But why...? I mean let's think logically. This card, with either unleash or Dreadscale, will achieve a 100% board clear. Either combo would cost you 10 mana.

The most comparable cards to that are:

1) Brawl - 5 mana. But this will leave 1 minion alive (and is obviously completely uselss vs. single targets)

2) Lightbomb - 6 mana. but this won't full remove minions with more health than attack.

3) Deathwing - 10 mana. Forces you to discard your entire hand.

So assuming you actually get the full combo (be it w/ UTH or Dreadscale) it is CLEARLY a better card than brawl or lightbomb. Meaning as a base - just for the removal, it should cost at least MORE THAN lightbomb.

So right off the bat you are at 7 mana, minimum.

Now, if you run it with UTH you are left with a 4/2 - or, roughly 3 mana worth of stats. And if you run it with Dreadscale you are left with an 8/3 - or, roughly 5 mana worth of stats.

So we're already looking at between 10 to 12 mana in value for a 10 mana play.

Then, you can't just ignore the "what if" your opponent can't remove it immediately? Being a turn 10 combo played when you are BEHIND on board, it is very likely you would have forced your opponent to play some number of weapons/spells/removal BEFORE playing this combo. As a 4 attack it's also immune from BGH and both priest shadow words (as opposed to deathwing which is easy victim to BGH/SW:Death). Assuming that is the case, it will be extremely difficult for your opponent to get back onto the board. That additional value can't be ignored.

So ultimately, I think the mana cost is balanced. The problem here is that the combo requires you to play 2 legendaries or a legendary + a 2-of spell, but it's in a deck with extremely weak card draw and the card draw they added (lock and load) adds "random cards" rather than actually drawing from deck.