Something is definitely up with installing the Plasma desktop, whether via the testing iso or on top of another DE.
Tonight I installed Budgie, which went well (still have the udev issue though), updated, then installed Plasma with this command:
sudo eopkg install -c desktop.kde.core
Here is Plasma after boot and trying to use the menu:

@Justin and @JoshStrobl do you know of any issues with installing Plasma from the iso and updates or via eopkg?

I've installed using the Plasma ISO about 4 or 5 times now and had no issues. Solus Plasma uses SDDM as the display manager. Maybe it doesn't like something in your hardware. If you're keen to eliminate it, it may work with lightdm.

    Justin SDDM worked fine as part of the whole iso on my hardware until the recent QT5 upgrade which broke boot on the LTS kernel.
    I took the opportunity to borrow my machine to test run Peppermint on that hardware for another project. @JoshStrobl remembers that part! 😉
    The SDDM problems have been since the full install after the QT5 fix.

      Hi, had the same issue when i tried Solus Plasma.

      Try this : before login in press CTRL ALT F2, log in in the terminal and run sudo nvidia-xconfig, and reboot

      (( Edit : you seems to be running nouveau driver, maybe you will need to install the proprietary one, for the GTX 260M : nvidia-340-glx-driver for the LTS kernel or nvidia-340-glx-driver-current for the current one, might also need nvidia-340-glx-driver-32bit ))

        Khio At this point I had Nvidia installed after installing Solus Budgie, with the 340.107 driver for my NVIDIA G92M [GeForce GTX 260M].
        I tried your suggestion:

        peter@erma ~ $ sudo nvidia-xconfig
        Password:

        WARNING: Unable to locate/open X configuration file.

        sh: pkg-config: command not found
        New X configuration file written to 'etc/X11/xorg.conf

        After reboot I still had the same screen issues, but at a lower resolution! 😁
        Think I am going to restart tonight testing fully with the installed iso, and test rebooting etc before running the updates.
        Thanks so much for your assistance.
        So glad it is my 2nd laptop. Hence why I have Budgie on my 1st!

          Have reinstalled from the Solus Plasma iso.
          Install went smooth, as did the reboot and login.
          I "turned it off and on again" and all is well... at this point.
          Have not installed anything, including updates or Nvidia.
          Heading to work. Shall continue this evening.
          (Note to self - check installed and repo versions of sddm)

          sudo eopkg info sddm
          installed = version 0.18.0, release 12
          repository = version 0.18.0, release 16

          sudo eopkg up = 455 packages to be downloaded and installed.
          Packages downloaded fast then a checklist ran, with the expected: Reloading udev rules ... failed
          Packages then installed fine, and another checklist was ran, with the same udev message.
          Ran a full shutdown and boot-up.
          In Grub, the default kernel is 4.20.10-111.current. Alternative boot kernel is 4.18.14-95.current, which came from the testing iso.
          Entered my password, and noticed that sddm kept reappearing as the desktop tried to load, and this is now what I am looking at:

          Would love to take a video as the screen is currently rotating between that photo and the desktop image as fullscreen.

          So, in recap, all I did was install from the testing iso, restart, download the updates, and restart again.
          Nvidia was not touched.

          Went into TTY and checked the info on sddm. Now on release 16 as expected.
          Fully checked the system with:
          sudo usysconf run -f && sudo eopkg rdb && sudo eopkg up
          Again, all checked fine, except udev, which I know I can solve by installing the LTS kernel, but this sddm boot-up issue is totally bugging me.
          Recently there was a QT5 update which needed a fix the next day. Has anyone else fully installed Solus Plasma since that time?

          While in TTY:
          sudo eopkg remove sddm
          sudo eopkg install lightdm
          sudo shutdown now

          @Justin let's see what happens when I power up again....

          Ok, that really didn't work, as it just took me to the TTY login, which now gives me a new puzzle!

            MintSpider Might need to sudo systemctl enable lightdm and/or sudo systemctl start lightdm and see what the second one says.

                Quick search suggests installing slick-greeter, so did:
                sudo eopkg install slick-greeter
                After shutdown and restart, I got to the GUI login, recognised slick-greeter from Budgie including the background photo, and booted into the Plasma desktop.
                However, I am still getting the same patchy graphics, and a menu system that is basically unusable.
                Giving up on Plasma on this machine.
                Something definitely changed not for the better in the update after installing from the testing iso.

                Justin Testing that now.... thanks for the input.
                You are an absolute genius... that was the part I was missing. Very much appreciated.
                Now the Plasma Desktop boots smoothly and the menu works as expected.

                Still, one should not have to jump through hoops like this.
                We need someone else to install Solus Plasma from scratch again and see how they do.

                    Back to the original theme of this thread: udev rules
                    Running Solus Plasma on kernel 4.20.10-111.current
                    udev not reloading is giving me 2 major issues:
                    1) Any software install takes ages, since this happens: [✗] Reloading udev rules failed
                    2) usb drives and devices are not auto-recognised. This has been tested with thumbdrives and gamepads.
                    Has anyone experienced this and/or have a solution?

                    Before reinstalling Solus Plasma on this machine I would run the LTS kernel and udev worked great.
                    However, unsure why, but the LTS kernel will no longer boot to a GUI login.
                    Note: I am running Solus Budgie on my main laptop (another Alienware M15x) with
                    the LTS kernel of 4.9.158-126.lts just fine.

                    I really wanted Solus Plasma to work for me again on my 2nd Alienware M15x laptop.
                    However, since recently re-installing multiple times I kept having issues.
                    Therefore, first thing this morning I installed Solus Budgie instead, ran updates, installed the LTS kernel, then Nvidia drivers.
                    All is smooth again on this machine.

                    Thanks all for your help, hearing me out, and advice, especially @Justin

                    For some reason Plasma no longer works for my hardware, but least I am still on Solus, and I love Budgie.
                    Now I have Budgies! 😃

                    @Justin check this out:
                    I tried again with Solus Plasma on another machine, my Dell Optiplex 755 PC.
                    It's an older machine, but least proves that the Plasma install does indeed still work fine.
                    Hoping to buy a new desktop this year.

                    peter@davie ~ $ inxi -Fz
                    System: Host: davie Kernel: 4.20.10-111.current x86_64 bits: 64 Desktop: KDE Plasma 5.14.5 Distro: Solus 3.9999
                    Machine: Type: Desktop System: Dell product: OptiPlex 755 v: N/A serial: <filter>
                    Mobo: Dell model: 0DR845 serial: <filter> BIOS: Dell v: A22 date: 06/11/2012
                    CPU: Topology: Dual Core model: Intel Core2 Duo E6550 bits: 64 type: MCP L2 cache: 4096 KiB
                    Speed: 1997 MHz min/max: 2000/2333 MHz Core speeds (MHz): 1: 2135 2: 2000
                    Graphics: Device-1: Advanced Micro Devices [AMD/ATI] RV610 [Radeon HD 2400 PRO] driver: radeon v: kernel
                    Display: x11 server: X.Org 1.20.3 driver: ati,radeon unloaded: fbdev,modesetting,vesa resolution: 1920x1080~60Hz
                    OpenGL: renderer: AMD RV610 (DRM 2.50.0 / 4.20.10-111.current LLVM 7.0.0) v: 3.3 Mesa 18.3.3
                    Audio: Device-1: Intel 82801I HD Audio driver: snd_hda_intel
                    Device-2: Logitech Webcam Pro 9000 type: USB driver: snd-usb-audio,uvcvideo
                    Sound Server: ALSA v: k4.20.10-111.current
                    Network: Device-1: Intel 82566DM-2 Gigabit Network driver: e1000e
                    IF: enp0s25 state: up speed: 1000 Mbps duplex: full mac: <filter>
                    Device-2: Qualcomm Atheros AR5212/5213/2414 Wireless Network Adapter driver: ath5k
                    IF: wlp3s0 state: down mac: <filter>
                    Drives: Local Storage: total: 465.76 GiB used: 6.40 GiB (1.4%)
                    ID-1: /dev/sda vendor: Hitachi model: HDS721050CLA662 size: 465.76 GiB
                    Partition: ID-1: / size: 146.65 GiB used: 6.40 GiB (4.4%) fs: ext4 dev: /dev/sda2
                    Sensors: System Temperatures: cpu: 43.0 C mobo: N/A gpu: radeon temp: 73 C
                    Fan Speeds (RPM): N/A
                    Info: Processes: 144 Uptime: 13m Memory: 5.70 GiB used: 1.05 GiB (18.5%) Shell: bash inxi: 3.0.32

                    Plus, no issues with udev and the current kernel.