For me it's not Goh himself that's the issue so much as what he does and how he does it and how many questions it raises about the series.
Namely why throughout the series has Team Rocket employed so many expensive traps to get pokemon if all you usually have to do is throw a normal poke ball accurately? For most of the series the answer was that catching a pokemon without weakening it first either required it to be a naturally weak pokemon, pure luck, or it allowing itself to be caught. Goh upends that for the majority of his captures and raises the question of why nobody else in the pokemon world seems to be doing what he is if its really this easy? Heck, catching pokemon in the actual Pokemon Go game is tougher a lot of the time.
Ash accomplishes a lot of things other trainers much older than him haven't, but in his case we get a lot of explanations (be it destiny, being something of a prodigy trainer, sometimes even luck), but Goh is just a fairly smart kid who decided he wanted to catch all pokemon for vaguely defined reasons (i.e. if he wanted to learn about them then actually raising them through their evolutionary stages would likely tell him more than just catching each stage) and somehow is able to get whatever he wants the majority of the time. Why is catching a ton of pokemon considered a noteworthy accomplishment if its as easy as Goh makes it look? Most of all, how can Goh's abilities grow when the only skill usually required is throwing a ball at a target?
Goh getting Suicune the way he did in this episode is fine in a vacuum I guess, but it makes one ask why keeping a legendary willingly around is such a rare event if all you really need to do is be nice to one and help it.
TR aren't smart though, they could easily be out there catching pokemon the legit way, but instead they are hung up on Pikachu and only stealing pokemon when Ash is around.
Destiny, prodigy and luck aren't exactly great reasons, Ash is portrayed as a hard worker and unconventional though. Over time the anime has been having Goh ape Ash's fighting style, though he is still clearly inferior. That doesn't explain his captures though.
We only know how on screen captures happened, but haven't there been a number of instant captures in the past: Bounsweet, Pineco, Cyndaquil, Tauros, Totodile (?). Darmanitan is an outlier in Goh's captures for being fully evolved and seemingly hostile. But in general, we haven't seen enough to know if simpler captures are something that is restricted to Goh or just a new take on the anime setting.
The majority of Goh's captures have been first stage. He only captured the lines for the Kanto bug's and Flygon, so I believe the writers are planning to have him evolve his other ones.
I don't think Goh's ball throwing is going to level up, he will likely continue to catch weak pokemon easily to evolve, and increase his battling abilities to fight either powerful single stage pokemon or legendaries.