A question about levels
So this is probably going to sound noobish but is there a reason not to fully upgrade a champion besides a lack of resources? I've noticed a lot of guides for act 4 and 5 say to have at least a (besides 5 stars) 4 star 4/40, but my 4 star awakened Hela has apparently the potential for 5 ranks. Is it just really expensive to max rank higher tier champions?