I don't think Tapu Tapu is going to be an issue. I literally HATE waiting in water park lines. Being crammed up against wet people on stairs and occasionally being dripped on by the slide is the most boring thing ever. I think/ hope that once they get the park 100%, Universal can focus on plussing the park with small additions like streetmosphere, a full service restaurant, maybe even a parade/water pageant of some sort.
If all you care about is running from slide to slide, what's the point of an all day water theme park? Real theme park fans don't enjoy the rest of the parks like that. You don't run to escape to Gringotts and then run out of Diagon Alley.
From the pictures we've seen the first two days, this is not Diagon Alley level, but I have confidence it can be. Even in its current state, it definitely deserves to be sipped like a fine wine, not chugged like a crappy beer.