It has increasingly aggressive swarms of tanks, helicopters, planes, and the classic Toaplan "box" enemies. It's meant to keep you on your toes and react quickly, though this is somewhat ameliorated by the use of the blue spread.
It's a different (and older) type of shooter that was by-and-large discarded by the 90s for flashier, more varied stage design.
Even the more "celebrated" PCE shooters are kinda bland by modern comparison. Most of them are content to repeat boring enemy waves multiple times without posing any threat to the player whatsoever (looking at you Burning Angels).
Though K-Tiger is a bit better in this regard: stages have actual level design and you are not just flying over a repeating background (clouds or sea) the entire time.