Infestation isn't a bad game at all, but it felt very mediocre to me. After the sheer gaming joy that was Contra 4, and then Shantae 2, which was also very good, I expected Infestation to be really good. The problem is really just that Wayforward set the bar too high with their past games.
I do have some minor quibbles with Infestation. The whole team mechanic was incredibly under-developed. You only ever see your teammates in the save rooms, and your different soldiers are just different names with no gameplay variation. It felt tacked-on and could have been turned into a really new, interesting concept.
The music was boring. The only notable part of the sound effects was the classic Alien soundbyte when the cat jumps out of the airduct towards the beginning of the game. The graphics seemed pretty bland, too. Most of the game looked like just another spaceship corridor or airduct. Not at all what I would expect from Wayforward.
I really didn't care for the way the game was divided up into different areas and you could only access the part of the game (Sulaco or Moonbase or whatever) that you were currently in. I realize this was tied into the story, but it's a little frustrating when you have this sprawling Metroidvania world, and when you get a new ability, you can only use it on the parts of the world that are currently open to you. You might have passed a steam vent in that last area because you didn't have the wrench to shut off the valve, and now you've found the wrench, but you can't go back to that area because it was on a different planet. That sort of thing.
I think I had some other issues with the game, but it's been awhile since I played it and I can't remember. But these are the most important problems. It's really not a bad game, just not up to the standards of Contra 4 and Shantae.