WonderJet

I'd say that compared to Rock Band and Rock Band 2, the vocals are less forgiving.  Harmonies are really truly a challenge, though!  My wife and I can both sing pretty well, but we've only been able to get all the double fabs in a song once, on hard or expert.   
 
On a related note (bad pun), I noticed that they somehow fixed the microphone lag.  In RB 1 and 2, I always have to start singing the note a little before the note actually happens because of the lag (even though the other instruments are calibrated fine).  It's the same delay that's audible during drum fills.  In Beatles, I don't have to start singing early.   
 
The awesome thing is how good you get at singing after working on harmonies for a while.  This morning at work I was able to sing Something and identify all the harmony lines in it.  It was really fun to be able to do that!