I liked Frank Miller’s art way more when he was younger than I did after he turned 30.

Stu Immomen's art used to be near-photographic... along the lines of Adams or Bryan Hitch... and then BAM he switched to a more cartoony style, around the "Agents of HATE" era.

Keith Giffen turned into a mess in the 90’s.

Keith Giffen for me as well, but 80's when he did the abrupt ripoff change. Besides the fact he got into all that trouble for imitating too closely another artist, I hated it on the Legion main title. When he was drawing "classic style", his stuff looked amazing, but the stylized stuff didn't hit the mark for me (though I was fine with it on Ambush Bug or the Legion of Subs special). In short order he went from one of my favorite artists to "thank goodness Steve Lightle took over".

Of course I disagree about Kirby.

Part of the problem with Ditko, besides his heart wasn't in it, was in his early days he inked his own work. This meant he left a lot out in his pencils and added it during inking, like Spider-man's webs. He really needed a "finisher" for his later work.
That said, I liked his work on Rom.

I think later Kirby work depends on the project. Eternals has material just as strong as the 4th World books. I think it was more the material he did at Marvel's behest, like going back to captain America or the Black Panther, where it falters; but, I still think there is a lot of great stuff in there. When he introduces Arnim Zola and that bunch, it gets weird as all heck; but, the visuals stick in your head. Once he goes of to animation, that power is still there, when you see his presentation art; but, age and health problems affect the sureness of his line.

Ditko I agree about the 80s and would argue the 70s, to a point. I think it is more his head wasn't in it, as much as his heart as the more Randian he became, the less power there was in the work. Maybe that's my own prejudices.

JRJR was the first thing that leapt to my mind. I was a big fan of his, on Iron Man. Part of the reason I dropped X-Men, aside from Claremont repeating himself was the art from Romita Jr. I really, really hated it.

Giffen has always had trouble developing his own style. Early on, he was very Kirby, with a bit more Adams, when he did stuff for DC. Kind of like rich Buckler. By the time he came back to things on Legion, with that evolved style, I thought his stuff looked fantastic, even as he ripped off Philippe Druillet motifs in some of his design work and architecture. Then, suddenly, he turned into a Jose Munoz clone, but without Munoz's power. Miller also turned into Munoz, right down to the subject matter, by Sin City. Look at Munoz's Joe's Bar and Alack Sinner and tell me that Miller wasn't drooling over it. As it was, he was also borrowing liberally from Goseki Kojima (Lone Wolf & Cub) and added some Moebius, in Ronin.

With Miller, it is more his writing that I find off-putting, than his visuals, though there is some of that in later work, like the follow on Dark Knight books. I don't rate The 300 highly, mainly from a historical perspective; but, his visuals are better there than DKSA and DKBADH (Dark Knight: Beating A Dead Horse).

Byrne is one where I loved his early stuff, then it seemed to get messier and messier and less and less detail, which I get, given he was writing, too; but, it just seemed less. When he could take more time and obviously loved what he was doing, like Batman/Captain America, I thought he could still bring it.

One where I would go the opposite direction (aside from the usual evolution due to experience) is Rob Liefeld.  He was the drizzling [email protected]#$% at the start (other than Karl Kesel cleaning it up) and continued that way through his Image and Awesome Comics stuff (and Marvel return); but, as the 21st Century has progressed, I have seen some really great art from him, with feet and everything!  I don't know if it was maturity or just having the time to do it right or develop it.