I upgraded from 39 to 40 and I think the only issues I had were:
Background I was using got removed, got a better one anyway
A few gnome extensions stopped working and I had to update them or find an alternative
Had to re-create virtual disk mapped to real disk for booting windows installation in virtual box (there is a sonicwall VPN I have to use for work which only works on Windows)
I think that was it!
I have had some strange behaviour from Firefox saying it’s become unresponsive a few times and at the same time Thunderbird but that seems to have fixed itself now