I definitely understand the critique with Goh since, as with many companions, his goal and arc sometimes feel like they're made up on the fly thus not done that gratifyingly. I do think they have improved as they went along however. I'd argue Project Mew is a more developed tournament idea than something like the showcases for example.
The 'Goh never loses' critique I'd also question since the reasoning behind that is the token format of Project Mew (which I'd argue would have worked well with several other competitions to maintain rivals as a threat even when they can't stack in loads of wins for each character). Goh has arguably had stronger personal losses than several companions, in particular ones that have labelled a flaw that he has to resolve or cases of him simply just not being good enough, both of which the previous Project Mew challenge demonstrated.
His trainer experience is questionable, but it's less in effect of him consistently being an incompetent that's prevented from getting any actual challenges like Misty, May or Serena, but more it feels completely out of order. One episode he'll be logical power level and even get the crap kicked out of him, the other he'll be a powerhouse. It almost feels like one side of the writing team wanna rush things and the other are saying 'no'.