anyway, what is his cheat exactly? it is just being skilled/smart?
The author's explanation of his ability/cheat is crap (a cryptic two panels about math formulae vs math operators). Basically, most mages in this world are like computer users. They can only use a dozen or so pre-made apps which are already installed on their computer. They can use only a dozen or so spells they were gifted with upon coming of age. And those dozen spells are all they can use for their entire lives. They can improve the spells' power as they grow, but they can't learn new ones.
MC is more like a computer programmer. He can use as many apps (spells) as he wants. And with a little work, he can create a new one if he feels the need for it. It's later revealed that his unique ability can
copy other mages' magic that he sees. So not only can he make his own spells, he can now add anyone else's spells to his repertoire. His chantless casting doesn't seem to be a unique ability, as it's mentioned he's teaching it to one of his party members.
This one actually looks promising. MC is not overtly OP at the beginning (he has yet to build up a massive library of spells). He's saddled with a pretty huge disability (rampant discrimination against 1-stars). And we have yet to see what happens when he hits the lower level cap of a 1-star. So this one at least has a chance of distinguishing itself from the quagmire of OP MC manga.
The main problems I see are that MC has annoyingly low self-esteem and is excessively self-deprecating. So much so that it actually causes relationship problems (i.e. his attempts to be nice and considerate actually creates problems, instead of smoothing over them). It is very frustrating to watch, and I don't read manga to get frustrated. And the translation quality drops a lot after about a half dozen chapters. (The English is readable so a native English speaker went over it. But the things characters are saying don't really mesh together, like there was no Japanese translator on staff and this was machine translated.)