I understand getting the overload, but if you overload yourself i think it's only fair to proc these cards aswell. Right now it feels like a double punish. This isn't gamebreaking, but i think its bad design of the card.
It's agreeably perhaps a bad design of sorts. Though one thing to note is the use of the words "When you play" which is probably how they are coming to that decision, because the spells are cast effectively by the battlecry of the minion, not by you. That said, you still take the Overload, which I suppose could be considered odd if nothing else. Perhaps this is intended from a balance perspective then, since those spells contain overload costs to keep them in check.
All that said, I totally agree that it's a little unfair that the spells don't proc the minion abilities.
they don't proc because the Drustvar horror casts the spell and not you. ooverload is part of the spell's effect so it's applied even when cast by another card's effect, Unbound Elemental and Thunderhead clearly state: "when you play a card with overload", not "when you overload", that's the key difference
would be nice if they'd change that (allso for the future). Just recently i did a heist with the shaman and his 1mana 1 overload draw a card heropower, which isn't a 'card you play' either, so unbound didnt work, but lickem did, since that only needs a crystal being overloaded
I think the key in this is that the cards you mentioned say „whenever you PLAY A CARD with overload”. Since only card you play is horror and he doesn’t have overload itself, Thunderhead and Unbound won’t trigger.