I feel watching AVGN and James & Mike praise the Pipeworks godzilla games set my expectations far too high going in, thinking they would be THE definitive godzilla hidden gem games. They're no doubt good compared to most of the other godzilla games, with plenty of cool kaiju-characteristic abilities such as picking up buildings and throwing them at enemies, but the singleplayer experience here is somewhat underwhelming. Survival mode is rather boring, Most of the challenge minigames save for basketball and ufo ones are genuinely painful, and getting points for unlocking stuff is tedious (apparently you have to beat the campaign with different difficulties and characters, i can't 100% verify, but i'm pretty sure on my 2nd playthrough as Megalon in normal difficulty i didn't get any of the unlock points i scored, then on my third playthrough in hard i did)
They're just fun party games for local multiplayer or for beating the AI, and that's it. Save the Earth is probably the most polished and content-rich of all the three games.

PD: I'm not usually a framerate elitist, but i heavily suggest avoiding the PS2 versions of these games. They play like fucking slideshows. I legit started hating this game a lot less after trying the xbox version with an actually playable framerate.