How deadly is boarding exactly?

Crew have to be killed 3 times over to kill a ship. A ship with "4" crew, has 12 crew dead before its a hulk.

An weak cruiser might have as few as 9 Troops per step. A strong one might have as many as 15.

That means even for killing a weak cruiser, a powerful boarding faction like Tyranids will need 3 boarding actions to instantly kill a cruiser. Even space-marines are unlikely to be able to de-crew a "weaker" equivalent ship entirely in 2 ships worth of boarding actions. An average faction will take 5 or 6 boarding actions to do the equivalent of two "good" boarding actions, so do remember here that the differences are quite huge.

That said, having 1/3rd your ships deleted is still somewhat terrifying, and even having 1 ship deleted from boarding can be concerning.

If a ship has boarding torpedoes, that will be roughly once more, its boarding ability. That means 3 Space-Marine ships can delete 2 equivalent ships including their torpedoes. Realistically, only half those will hit. That said, they cannot be entirely blocked off by fighters, making them more resistant to counters.

If a ship has assault boats, each one is capable of a boarding action, but they are much harder to ensure a sufficient number of hits

How much can they kill?
This means, that space-marine LCs can, if not dealt with, kill about 2.5 enemies on a good run, for every 3 of their ships.

This is why you have to ensure that you deal with boarding torpedoes and assault boats. 1/3rd your ships get killed off, you can recrew them with another 1/3rd, and then maintain some function. If over half your ships go down though you are entirely dead. Its a very slippery threshold here, and the 'potential' damage is one assault instantly removing you from the game.

This is the range of boarding effects in the game, and why thresholds matter so much. Because you can recrew, or recover some troops, smaller thresholds become radical differences in ability.

What this means for you
For players, one should keep in mind the practical effects of this. If you are being lightning striked you are probably fine. If you are being boarded at close range, and the enemy has other tools - this is how boarding will instantly kill you.

From a design perspective
This means that heavy-assault factions have little reason to care about guns, as guns support boarding little, and boarding supports guns little. The threshold for recrewing ships is quite small, and will chain sufficiently. Likewise, at short range if you are losing against opponent's weapons, you will be shredded before you get another volley off unless you have sufficiently crippled the enemy.

I do think the current design pattern for core boarding is about right though. At most 2 ships can de-crew an equivalent enemy, and realistically it takes 3. This is about right, all things considered. The problem is that ordinance allows hyper-specialization thereby invalidating other choices. Incidental boarding power is interesting, and a powerful faction ability. When you allow people to specialize, you then allow people to refine their win metrics around it only, and this can easily become the only way the faction can win.

This is the case for both space-marines and Nids, because they are allowed to have massive focus in boarding, some spacemarine LCs literally having a majority of their weapons being boarding related. For both factions, I would actually recommend making their assault boats and boarding torpedoes worse than usual, and strengthening other options. An average option in those areas is already dangerous, because of the advantages of specialization. A better balanced system would be to balance around the Razorfiend or the like, and grant real disadvantages for extreme specialist weaponry. The powerful boarding that both of these factions have base is enough, forcing specialization into it requires more robust systems in place than the game currently has.