I think an error on return of 2nd serve can most certainly be treated as unforced, but I agree it is so subjective and alot of times needs to be taken with a pinch of salt. I also thought for some reason that aces and dfs were included as winners and unforced errors but apparently not.
It seems to me that it varies from tournament to tournament. I have definitely seen aces counted as winners and not counted as winners. My memory isn't good enough to specify which tournaments.
Yes, and I think that's the point. Different tournaments use different systems and even within that different scorers can score shots differently (not aces of course).
It makes sense to me not to have aces as winners and DFs as UEs simply because it looks like double counting - 3 aces and 5 winners, to me, reads as 8 'winning shots', not 3 aces and 2 more winners. But I could get used to reading it the other way.
But I agree with Phil that I never understood the policy (which some still have) of never scoring a missed return of serve as an UE. A doddy second serve that you dump in the net is almost certainly an unforced error, nothing forced about it. First serves are more tricky (more likely to be forced errors, not unforced ones) but it's all a question of judgement so definitely arguable.
Gee, I was always aware of some subjectivity in UEs ( really impossible to avoid some element ), but you'ed think they would make more of an effort at consistency.
And such as different assessment of aces and DFs is plain annoying.