As a brief taste:
* No trackpad swipe gestures (plus many more subtle trackpad issues)
* Pre-Wayland the only way to attach a second screen at a different DPI was with xrandr that caused random flickering
* Accidentally bricked it once because I restarted while updates were happening in the background (no indication they were, and no prompt to let me know before restarting)
I could go on for awhile...