Again, I think this is the "class legendaries must be win conditions" mindset. Bolvar is 5 mana, not 7 or 8. Velen, Antonidas, and Kel'Thuzad also all need combo setups or specific board states in order to work. The only prerequisite for Bolvar is that he might have to sit in your hand for a bit. Once he gets to the target you need, you can drop him You don't have to wait until you draw a mind blast, a mirror image, or you start your turn with a sludge belcher on the board. Just plop him down and you're done.
That still makes him worse than pretty much any other 5 drop though. I can drop a Loatheb at any time, and he'll be good. I can drop a Sludge Belcher or a Spectral Knight and they'll be good. If I top deck a Bolvar he's just an overcosted Mogushan warden. He requires a significant amount of effort just to be mediocre. The times he'll even be better than Feugen are going to be rare.