Overdrive detection on vocals is busted, i have hummed, yelled, whispered, blown into the microphone when there are small bars in between verses and it never registers, the entire community has complained about this as well.
The initial update on XBL/PSN did a lot for this but it's still fickle
The HOPO timing became extremely tight, i literally had to learn how to play again just because of those, now i have them down again and its not so bad, but at first i was getting pissed that i was failing songs when i was certain i hit the notes.
The HOPO timing like OD activation on vox was initially problematic but got expanded a little with the initial update (I suspect it may have been deliberate to screw with the people who got it early). If you're still having issues with the identical-to-RB2 timing window, you should probably adjust your calibration settings (most people on ScoreHero say somewhere around +20/+20 compared to your RB2 values should work pretty good).
Also as a note, All Instruments Mode is not required to use the keyboard accessory. There are two large reasons for All Instruments Mode working the way it does.
First, the 360 and Wii do NOT properly support connecting more than four controllers at a time (and in the 360's case, it definitely doesn't support more than four XBL logins at once via the dashboard), so "screwing" one controller out of five was somewhat of a necessity. The lack of even a % is a bit of a letdown but it's a necessary evil to make guitar+bass+keys work.
Second, online play. There are 2,000 songs that don't have keyboard parts. If you could have g/b/k all at once online, you'd either have one player sitting out
constantly or needlessly restrict the available songs to a very small selection just because a keyboardist joined in alongside your guitar & bass when you wanted to play older songs. With this system, the keyboardist can improvise on guitar/bass parts (with mixed success lol) for non-keys songs.
The mechanics of All Instruments Mode is sadly a necessary evil this gen. Hopefully next gen will support more controllers better.
there's actually one more reason... AIM also forces the venue into "music video" mode because Harmonix's engine programmers aren't talented enough to get the engine to support a 5th player on the venue stages... but the gameplay issues make this largely irrelevant when you look at the big picture