Hau honestly suffers quite a bit from Show, don't Tell. Gladion tells us that he has insecurities around his relationship with his grandfather, but we never see that type of relationship shown between the characters, he's supposed to want to surpass his grandfather, but we don't see him make the progression to surpassing Hala, and he's supposed to have taken Lillie getting stolen away pretty hard, but it doesn't visibly affect his mood or behavior throughout the main event and him taking the loss seriously is subtle at best. The fact that we also don't get to see him beat Hala is also something I think is a missed opportunity as I think having that moment in the game would've meant a lot for the character and we could've seen how that success affects him as a character, but we never get that and the only time that really is alluded to is a line from Hau at the league that you may very well not see if you don't rematch the Elite Four. Contrast that to Lillie, who's relationship with her mother is given more onscreen depth and we see the character change from being under her mother's thumb to actually calling her out for her own flaws and behavior.
Admittedly, this is a fault of Pokémon's storytelling in general as they rarely give that type of focus to other characters aside from the protagonist, but I think Hau did suffer for it as his good and interesting traits aren't given as much time and focus to really be strong as a character. I don't think he's terrible, but I do feel of the main characters, he has the a lot about him that is underdeveloped and underutilized.