The problem with the Tyranids is that they seemingly have no weaknesses.
They kill you within moments as soon as they reach you, and they can hulk your ships from across the map with their invincible boarding boats. They're survivable enough to simply shrug off fleets pounding on them and have decent speed combined with a boost that can more than compete with the best of them.
Not to mention that instead of being a horde, their fleets are more often just 1-2 large ships which shrug off anything that can be thrown at them.
I would suggest that the Tyranids receive pretty large nerfs to their manoeuvrability (especially the boost), survivability and have their assault boat spam massively reduced. In return, the Tyranids should get slightly more accurate bioplasma cannons (which have terrible accuracy right now) in order to make them more rounded and a decrease to the cost of their lower tier ships in order to really make them feel like a massive horde of individually expendable organisms.