From what I can tell the mechanic is so they can not react against the attack that you throw at them, but recently (I don't know if it's always been like this) guardbreaking in general gives you one free heavy. This makes the throw down in my opinion useless. In addition the whole free heavy thing is annoying, I just recently watch my teammate Orochi vs the enemy Orochi and it was a match of who could smash that guardbreak button faster.
Raider can either get a light into unblockable combo guaranteed hit or can do heavy into unblockable mixup and since they are exhausted they can't feint their parry attempt and you can parry them and knock them down to start it over again. Shug gets a fully charged over head heavy or a guaranteed demons embrace and conq can get a charged heavy off. Kensei gets a guaranteed unblockable, warden gets a top heavy, valk gets a guaranteed dash attack,light and sweep into a heavy, warlord gets jump attack into headbutt light, lb gets a light into a heavy into unblockable mixup. Someone already mentioned zerk and orochi, not sure about nobushi. Pk can get two stabs, then knockdown into a heavy into a bleed. All far more optimal than just gb heavy.