All in one pellets are generally better because the rabbit can't selectively feed the bits it wants to eat, but also I think to a degree the muesli mixes are created to look nice to us- all different coloured pieces of stuff so we think that's a nice looking variety mix. Some of those bits, when looked at closely, aren't nutritionally great for rabbits, like maize kernels and bits of sugary fruit.
I used to use Burgess Suparabbit but my previous bunny Oren used to get tummy troubles from it (I've heard of this happening occasionally in some rabbits, I think someone said it had a higher sugar content? Not sure...) so now mine are on Science Selective instead and he never had a problem on that.