I play expert guitar and bass always, and I keep no fail on even if I don't need it 99% of the time. It's to a point where the few songs I need it for, it's usually only for small parts, so it lets me practice everything.
For Drums, generally there are 3 problems I have with playing an expert chart (which I play on no-fail).
1) There are small sections I can't hit. If they're not too prominent, I'll just live with it and get better.
2) I can't hit some section of the main pattern. If it's enough to simply fail me quickly normally, I'll drop to hard. If it's not, I'll try to power though it and learn what I can. Depends on how complex and fast it is.
3) It's just too darn fast. I've gotten a lot faster, but some songs just kill me with speed (Lonely Boy even on Hard for example, or Walk of Life) I'll try to keep up, but if it's too fast, it just wears me out (which prevents me from playing even more), even if I can hit it for spurts. Usually I'll drop down.
Guitar tends to have a lot less repeated patterns in my experience (or at least, the patterns are longer) than Drums, so there's a lot less of me being able to either hit it or not.
That said, I find that moving around between Guitar/Bass and Drums helps me out a lot, as often as I improve with 1, I improve with the other as well.