Blizzard have commented on this afew times, what you’re proposing is removing the pure RNG mystery from mystery heroes.
It’s a highly debated topic both on here and from what Jeff Kaplan has said in the past also in the office.
Currently it is purely mystery, purely RNG to it all and that’s the point, some love that (I do personally) others don’t, others want some kinda mid point (no more than 2 of each hero etc).
It’s not broken at all, it is what it’s intended to be a mystery with no rules to it, adding rules to it means it becomes ‘semi mystery heroes’ ie another game mode so yeah.
As Jeff Kaplan posted afew weeks ago, maybe they will do ‘Semi balanced Mystery Heroes’ one day, but not atm.