First of all, a turn 5,6,7 etc mountain giant doesnt cost 4 mana. It costs 5,6,7 etc mana in the specific scenario in which you barely played any cards for the turns before that, making it a bad play tempo wise.
A mountain giant only ever costs 4 mana in a warlock deck that passes every single turn before playing it, ignoring what your opponent is doing.
So there is ONE specific scenario where a mountain giant costs 4 mana and in that scenario it is very likely that the opposing 2 and 3 drops are still alive on the board - because you cant have killed them while still being able to play the giant for 4. So in that OPTIMAL scenario, the opponent is just going to trade his 3 drop into your giant + a spell and its dead.
Or, what is likely happening, he BGHs it and now you face 3 minions while having an empty board. OP amirite.
And yes, it makes handlock unplayable. Dont argue semantics here, ofc you could click handlock and play with it, technically, but the fact that so many decks run 1-2 BGHs means that you conciously decided to play a deck thats going to default lose a lot TO ONE SPECIFIC CARD.
A rag kills an 8-health minion IF it hits it, obviously needing the specific scenario that the 8-health minion is played prior to that. Dr. boom can not deal 17 dmg the turn after its played, thats another scenario thats less likely that winning the lottery.
BGH kills a 7+ attack minion. It doesnt MAYBE hit it, it has no rng, it always hits the minion you want it to hit, it always kills it and it does so for 3 mana.
The amount of cards that are not viable simply because BGH exists is insane. Malganis is unplayable because of it. Thats hilariously stupid.
The simple fact that BGH exists makes 6 attack minions STRONGER than 7 attack minions, how do you not see how fucked up that is?
BGH is so stupidly broken that it even hampers new releases FOREVER. We will NEVER have big minions that are usable as long as they dont have a strong battlecry or deathrattle.
Like have you actually checked how many cards, just in GvG, are unplayable because of BGH?