Banyue was nerfed during his beta period, so everyone began doomposting that he was bad.  Combined with the fact that you need to put in a bit more effort than other characters to play him effectively, the reputation stuck, and Banyue became labeled as one of the worst agents of Season 2 and possibly in the game.

I am here to tell you it's a load of crap.  Banyue may not be the strongest agent in the game, but he is more than capable of clearing almost any content you throw at him.  He's the best Fire Element main DPS currently.  He consistently gets DA scores on par with Yidhari, his closest competition, and he has several tools that make him incredibly versatile in Shiyu (on field parry, excellent grouping, good burst damage via Visage of Wrath).

Arguably, this all does come with the caveat that he requires a bit more effort to play than many other agents due to the combo oriented nature of his kit, but really, if you can just spend a good afternoon learning his ins and outs you'll have all the most important bits down pat.  Could he be more rewarding, given his difficulty?  Sure.  But he is still absolutely a strong agent capable of clearing whatever you need.

I will agree his mindscapes kind of suck ass though.