If WotC were willing to risk permanently and dramatically changing all future games of Magic by introducing Companions, then there shouldnt be much of an argument about making single-use, bankable mana tokens a part of most every game going forward
My goal today is not to diminish any of those feelings
That is the prime directive, and considerations such as dominance, archetype balance and anything else should be only secondary in nature
Most combination decks had at least one card they couldn't win without, many had multiples and even the cards they could win without could slow things down quite a bit
All of Them I could quibble over Signets, Talismans, Fellwar Stone, Mind Stone, and so on, but truth is all the 2-mana rocks basically play out the same regardless of extra utility
Compared with other removals, Unholy Heat does therefore not see as much play as Lightning Bolt or Prismatic Ending and this is the reason why I did place it just outside the Top Five personally