On my computer that’d unmount my home directory, my external storage, my scratch space and my backup storage, and my NAS.
It would also unmount /sys and /proc and /tmp and /run. Things can get weird fast without those, for example that’s where the Xorg/Wayland socket is located.
If all you have is home and root on the same partition I guess it’s not too bad because it’s guaranteed to be in use so it won’t let you, but still, I wouldn’t do that to save like 5 keystrokes in a terminal.