The main issue with this card is that the effect is battlecry, not ongoing, so it can be easilly removed when it hits the board. There are many cards in wild that can make it 10/5 as soon as it hits the board (turn 3 unlicenced apothecary, turn four crystllizer + buster = 11/5. That said, the ongoing effect would be broken in wild, so i guess that's the reason its bttlecry.
High roll all or nothing card which makes it very unreliable. Not to mention that most decks will be able to remove it on turn 4/5.
I tried the card when Boomsday was new, but it was mostly disappointing for me. It's good if you can activate it exactly on turn 4 (or 3 with Coin). If you can't activate it you'd much rather have Tar Creeper. Activating it on turn 5+ often has too little impact: for example on turn 5 you'd rather play Despicable Dreadlord than tap + Nethersoul Buster. So there is a really small window where the card shines and for me it didn't shine often enough.
It's a pretty good card that doesn't see much play at the moment because it doesn't make the cut in healzoo, and healzoo does the same thing Nethersoul Buster does only better - i.e. dump huge amounts of stats onto the board very quickly.
If Zoo is still a thing after the rotation, it may very well see play then.