well, zac is really strong right now, almost as strong as gragas and sejuani, he only has two downsides, one of which is minor, one of which is major.
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in duels but even more so in teamfights, he isnt able to consistently pick up his blobs. this way, he loses alot of HP during an extended fight that would otherwise be preserved on other champions. this DOES affect him and weaken him in big tamfights, even tho it buffs him ridiculously in skirmishes and jungling. tho since sustaining in jungle is no longer a problem this meta, it doesnt matter much.
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the major downside tho is that none of his abilities is strong the moment you press a button. he is somewhat sustained magic damage and "sustained cc" if you wanna call it that. he has no burst and requires at least not-retarded use of his abilities, while a gragas or sejuani can press all their buttons and their teamfighting is done. all abilities just do something and it all has a pretty big effect on its own. zacs biggest damage potential comes from his unstable matter and he has to pick up blobs and spam it on many people to be impactfull, but since picking up blobs in a 5on5 is unlikely, he loses out on not just healing but also damage and his ultimate is sustained too while its CC is actually low for todays standards.
like, gragas and sejuani are really the prime examples of "press button, be effective regardless of brain damage suffered during childhood abuse". zac doesnt have that, so even tho hes really strong, hes not super easy and freewin. tho im also not saying hes exceptionally hard. i think hes pretty much good where he is now, tho i would nerf his jump as in you have a smaller area to chose from aswell as a limited range choice. like when you charge it fully, you cant jump melee range anymore and such, because i think even tho the jumps range and damage and cooldown is all good, it gives a bit too much freedom of choice.