For the purposes of this, I am going to often compare Tyranids to the closest alternative option, Space Marines. They are very comparable for many reasons, having high morale, powerful boarding, and weak weapons.
To start with, what are some strengths of Tyranids?
6.1 /12.2 /18.3 Average Boarding damage. It costs about 16 points of ship to deal 1 point of troop damage.
Compare to 8.7 / 16.8 / 25.1 Boarding Damage for SM. Its more complicated to measure, but is closer to 12 points of ship per incidental troop damage. SM also get more critical damage from boarding.
4000 Range Lunge on a 45 second cooldown.
120/200/160/160/160 Speed (+88 if using lunge on cooldown)
240/240/200/160 for SM
25s of 4x speed. with 100s cooldown. Meaning average speed is double for SM when using their boost on CD.
12/12/8/8/4 Rotation Speed
20/12/8/4 Rotation Speed
Alright, these are the main 'stat' categories you would think they might be better in. Sadly that is not the case.
When it comes to blitz boarding, SM are better than nids by about 25% in nearly every way. I have not included here the power of boarding torpedoes, or that SM additionally get use of lightning strike, or that SM are immune to lightning strikes when shields are up and nids are not.
So then, in what way are nids better than SM?
A 200 Pt cruiser has about 10 effective DPS for SM and 16 Troops. The Nid Cruiser has 18 troops, and 14 dps. A huge increase in DPS. Additionally, its critical hits start fires for more of an advantage. Likewise there is no way for SM to really deal comperable damage, though they can heavily out-range the tyranids, by double - granting (even including a reflexive and somehow instant charge) 25 seconds or so of fire of the nids, before they can close - if the Tyranids are scouted first (which is not hard, but might suicide an escort or some fighters).
25 seconds is only 250 damage for the spacemarine though. That is about as much as the Tyranid shield advantage on spacemarines. Sure condensed fire might cripple a ship before it gets in, but this situation is not that bad.
That said, the Spacemarine armor advantage means that while the Tyrnaid ships have roughly 3600 health, the spacemarines have roughly 6000 health. Not actually enough to mean that the Tyranid ships are out-DPSing the spacemarines.
There is one advantage you have - feeder tentacles. With a range I estimate to be 6750, and dealing up to an additional 6 boarding damage, feeder tentacles push escorts far above the average for crew damage, and devourers slightly above the SM average (but in a frontal arc).
By my sincere estimates, this is the one way nids have any advantage over spacemarines (boarding torpedoes can in theory be negated even if not in practice).
I was actually expecting to find more advantages for them, and it is true that Nid carriers are slightly better than spacemarine ones, but the spacemarines also get bombers as an option, so I cannot even in good conscience flaunt that. Nid ordinance is also slow, and therefore could not be called a strict improvement over spacemarine options.
I will perhaps edit this later, but I am pretty disheartened. I thought I would be able to find more that Nids could do, but have instead just found even more weaknesses than I was expecting.
Tyranids have Regeneration. A powerful ability that heals critical hits automatically in 20 seconds. They also have powerful feeder tentacles that put them slightly beyond base Space Marine boarding stats. Shroud hull is also nice, though it is inferior to LC native stealth ability.
In return, they only have to suffer worse brawling ability, turrets that can be destroyed with disruptor bombs, instant death on de-crew, no ability to increase their weapons range or accuracy like all other races, no lightning strikes for longer range boarding and the slowest ships in the game.
I will format this to be better later, but am not feeling it after reading all of those stats.