As much as I sympathise with developers over headaches caused by themeing, I believe users (myself included) would be less up-in-arms about it if applications were less ugly by default. And boy howdy is libadwaita ugly as sin.
I'd be perfectly willing to tolerate a mismatched system, if the individual components looked at least okay. Like I'm not going to get e.g. steam or discord to match an e.g. Windows 9x theme, and I'm mostly okay with that because they aren't horrible to look at. But, say, File Roller? Absolutely not. Horribly ugly.
If that's the best example of criticism to be had about COSMIC, then it's practically flawless. Except of course, though, it isn't. Aside from the general lack of polish, I'd argue that there's not enough customization. E.g. things that should really be a slider or spin control (or better yet multiple sliders/spin controls) like corner radii are multiple choice button things (not to mention that the 'square' style is anything but.) Some of the sliders that do exist (particularly the size ones) have very few set points that make them essentially disguised drop downs. I could probably find dozens more things to criticize if I cared to sit down and nitpick everything.