I thought RB3 was kinda weird? The art style made everything feel really plasticy, it was really hard to have any guy characters that weren't buff monsters. Other than that, the vocals engine really threw me off, harmonies are neat but kind of a weird division that felt like it was dividing the scorehero vox playerbase. A lot of people chose Harmonies and just seeing how good they could do at that on their own, but I always liked solo vox more cause it felt more genine? Idk.
Mostly, I felt the addition of Keys was the biggest problem. I never had a keyboard, so I don't know how it played, but they didn't really make room for it in the game. Like the core concept of Rock Band is everyone working together, but the game still only allowed for 4 players. You could do all instruments mode offline, but that left out the vocalist(s) and if you don't get points then boo. Not to mention even in that mode only four player characters are represented on stage. Plus, online vocalists were still a staple, so it was usually deciding to cut out keys or bass. And that really sucks, cause like. That's just dividing the band and kinda goes against the spirit of the whole series.
The DLC was okay I guess.