This review <https://www.theregister.com/2021/12/16/pop_os_2110_new_system76/> of the new version of the Ubuntu variant from System76 mentions a lot of trouble getting it to play nicely with other installed OSes on the same machine. One key difference is that it doesn’t boot with GRUB. Instead, it is EFI-only, and requires your kernel and initial RAM disk to live inside the “EFI System Partition” (ESP), where they can be found by systemd-boot. This typically requires the ESP to be enlarged beyond its initial size. And, depending on the tools you use, that can be problematic. This EFI-only boot procedure can also complicate coexistence with other OS installations on a multiboot system. But, interestingly, not Microsoft Windows, at least according to one of the reader comments. Indeed, a well-known issue where installing Windows on an MBR-based system destroys your ability to boot other OSes goes away with EFI-only booting. Seems Windows is well-enough behaved to only replace its own EFI state, leaving that for other OSes alone.