mdiemer I agree it is not good to bash other systems,
+1
mdiemer it is a fact that their grub can cause problems in multi-boot systems.
Manjaro grub can boot other OS's grub well. It's just that other OS grub cannot boot manjaro (without modification) and the reason for this is the way manjaro handles intel-ucode (and amd-ucode). Intel-ucode is not incorporated into the kernels, rather as a initrd file and needs to be specified in grub together with the kernel initrd file to boot.
Other OS grub, following upstream grub, will just pick up this intel-ucode initrd file and not the kernel initrd file and hence will boot manjaro with a kernel panic. But yes, that's the way upstream grub works and nothing wrong with that.
Some may not like the way Solus handles uefi bootloader too. I personally think not able to install Solus to a separate $esp is not good if the existing $esp is a already in /boot/efi (for grub) and Solus needs $esp in /boot. And Solus bootloader does not pick up other OS's though it doesn't mean we cannot boot other OS's. Just not through its bootloader (though in a complex way, it can be done). And I personally think Solus can just make its bootloader fully systemd-boot as clr-boot-manager is just a stripped down version of it. Lastly, for multibooting, nothing can beat grub2 and Solus should allow implementation of it. Most distros, while having grub 2 as default bootloader allows implementation of refind or systemd-boot. Linux is all about freedom and choice and that is something we should cherish.
Cheers.