My problem with Go is pretty simple. Whenever the focus is on Go himself and his interaction with other people and just him living life with Ash, Koharu and the others, he's handled pretty well and is a likable and understandable character. His best episodes are the ones that focus solely on him as a person instead of him and his Pokemon. Heck, his best episode is probably still the one with his old childhood friend, even if the reason behind the strife was silly lack of communication but, you know, people. We do that.
The moment that focus shifts to being about Pokemon is when he becomes a haphazard and irritating mess with so many inconsistent random happenings and moments they try to force as being impactful and emotional when all the proper elements and set up needed just isn't there and the moment just doesn't stick the landing, but he cried a little and looked cute so some folks will eat it up. It feels shallow and hollow. I can't bring myself to like any of his Pokemon related focus because I very much don't care about it. The show and the character has done nothing to make me care about him catching tons of Pokemon easily or eventually getting Mew. It's never interesting or cool looking to see him do it or talk about it unlike Ash's battles and captures.
There's also the dilemma of his "goal" begging the question of "and then what?" in regards to it's completion. If Ash battles and beats Leon, what will he do? Keep traveling and having more adventures and battles on his quest to become Pokemon Master. His goal is always more about the journey than the destination because there's always more journeys to be had. Same for his other companions when it came to contests or whatevr else they wanted to do. Even if they were able to reach the goal, there was always something more past that goal. Go doesn't have that. He wants to catch Mew. Catching all of the other Pokemon is just an excuse to give him something to do until he does and is more of a means to an end rather than being part of the plan. Without catching every Pokemon, he'd just stand everywhere doing nothing since they aren't actually playing up him being smart and knowledgable. If he gets the chance to catch Mew without having to get them all, he'll do it. And then what? If Go, by some miracle, actually catches Mew, then what? He doesn't like training or battling and isn't great at it unless the show dumbs down others around him to make him look better like in that Flygon episode. Otherwise, he usually has to get saved like in the Gary and Dawn episodes. He doesn't actually study and research Pokemon outside of talking to his Pokedex liked everyone else, so I couldn't be interested even if they claimed to go that route. Just catch more Pokemon even after getting Mew? Well, sure, okay. They can do that. It won't be very interesting to watch if he doesn't get some battle skill, though. Pokemon GO style catching is boring to watch in a TV show.
Go the character is fine and a relatively harmless guy who's a joy to watch when he's just living life with his friends. Go the Pokemon character is mildly annoying on some days and incredibly frustrating on others, but is consistently pretty dull and boring to watch with an uninteresting goal that has a finite end. Honestly, both Go and Koharu feel severely out of place in the Pokemon universe and feel like characters that were pulled out of some "coming of age" slice of life anime and got isekai'd into Ash's Pokemon world and somehow changed how that entire world worked in the process upon arrival.
In short, Go is an okay anime character, but he's a terrible Pokemon anime character.