I shot 3 under last year, but went back to split because I feel I have better control of the arrow and the shot is quieter. I don't really anchor any differently, the arrow is just a bit lower. I shoot instinctive, so the arrow position didn't really make that much difference. At first I wasn't as accurate, but after a while I can't tell any difference. With 3 under, I always had the feeling the arrow was going to escape at an inopportune time, like when a deer was under my stand.
By the way, Byron Ferguson recommends starting new archers 3 under, then having them switch as they gain proficiency and confidence.
As many have stated though, there is nothing inherently wrong with staying 3 under if it works for you.