While Tyranids are quite strong in the moment, the entire idea that they shouldn't outdps other factions is wrong. Tyranids have plenty of ships whose only purpose is dakka, otherwise they wouldn't have guns.
In addition, we are talking of mostly short range firepower on the slowest faction that also cannot focus fire. You can alternate ships to protect damaged ones simply by moving them slightly behind your other ships.
The issue is not pyro acid itself, you don't see acid Voidprowlers or acid Razorfiends.
The issue is 2 particular ships: acid Hive ships and acid Krakens. Technically also caustic Drones, but those are the slowest ship in the game and quite fragile so it balances it out.
But the rest of pyro acid ships are fine or even still quite weak.