an old issue back when i started using linux if you had used kde and gnome (but this is ages ago back when kde4 existed)
skype and all QT or KDE apps would get 2 px font on 1 DE but not the other even though your settings said 8px or idk 22px, this would persist even if you completely removed the other DE, some gnome apps would just not work properly or be slow.
otherwise issues that can still happen:
settings for KDE or GNOME will not always match what settings actually say they should be in some cases.
and if you removed your previous DE sometimes the system would still try open files in the uninstalled applications because they were set as default (luckily this is an easy fix but a new user wont necessarily understand that) and not updated.
essentially if you rebase between DEs you can end up with issues that would require manual fixing and we are not interested in supporting that.