They're kinda fun gameplay wise honestly, but they effect the game design in a weird way because since suddenly every vert air can combo into a revert and manual and continue nearly indefinitely if you're good enough skill wise and your skater has high stats, some of the high score challenges and combo goals start getting kind of ridiculous to compensate the further into the games you get.

I vaguely remember one the last parts in Underground on sick difficulty has you do like a 2,000,000 point combo while you hit like every gap/obstacle in the New Jersey level where I was just like "haha this is bullshit." (I know there are really good THPS players that can do 9 figure combos routinely, but it's a really steep difficulty curve for a broad audience.)

2 has manuals to link street combos, but vert airs are basically reserved for big spin+flip+grab airs and high point special moves, and the high scores are more about creatively navigating the levels with manuals between grinds and getting multipliers from gaps instead of just doing "robot lines" with like 50 reverts anywhere. Some of the later games' additions like double/triple tap trick modifiers, flat ground tricks, grind modifiers, spine/hip transfers improved elements of the gameplay, but they kinda jumped the shark with Underground imo with on foot sections and gimmick vehicle bits. From a gameplay design standpoint, 2 is the most focused and polished with the least clutter.