It is a minority of my two's diet, they eat loads of hay, even still the muesli I use I think is a high-ish fibre one for muesli (RR), they get veg and herbs and the odd bit of fruit, so even if there was a lack of a certain vitamin/nutrient in their feed they are getting it from the rest of their diet. But RR is good quality and has a variety of natural and healthy ingredients so should contain everything they need. As long as I feed them not too much they don't selectively feed. If I gave them way too much I reckon they would, and they would probably not eat as much hay either, but the I think this is the same for pellets, if you give too much they would go off their hay, they just wouldn't selectively feed. I think if you feed the correct amount of food it shouldn't matter so much what it is (as long as it doesn't have something that could actually harm them like too much calcium).
Also, my two are healthy, happy, the correct weight, have shiny coats and have never had gut problems (ok well Harvey had two bouts of stasis last year, but one was after he fell down his ramp and hurt himself and the other was after I have him too much of a new vegetable, so not muesli related!) so why would I risk changing to a different feed which may (however unlikely) cause them to lose weight, gain weight, get gut problems or choke when I know the feed I have works for them? It seems like a strange thing to do to change their food when they are healthy on one! We all know it can upset some bunnies tummies to be on new food however gradual the introduction of it is!