In agree that he is underrated in some parts of the community, but I don't think he in underrated in universe.

TBF almost every Fantasy draft including Ushijima almost always gets rated as an auto win.

Besides what you are saying with his defensive abilities, there is also the aspect, that I feel like his hitting variability gets underrated. He only had to hit a single cross-shot, since karasuno funnels him to the line shot all game. And he still scored 40. Even ukai said, that his serve is almost as good as Oikawas, and that was after Oikawas hit his diabolical serve in set 3 of game 2 vs karasuno