In my opinion, I think the rewards are pretty fairly spread, it's just getting to those specific tiers that I don't always like.
I made a point a few months ago about raising the sacrifice value of 3* toons to a high enough amount where, if you could fully level up and evolve 10 3* toons you could at least get a tier 5 or a very high tier 4. Otherwise only 4*s can make the middle tiers and they're really meant for the high tiers.