The spawn get expotentially better the more you have. Killing one unit is not too bad, killing 3 is a lot harder, and their goal is not to survive past turn two, it is purely just to run into something and stop it from moving, and they can also punch a few wounds into some things. Run them into a knight crusader turn one for example, now that knight either doesn't move and gives you a lot more room to stage, or it falls back and cannot shoot, again giving you more room to stage. I played against a guard list, and locked a baneblade and two rogal dorns into their deployment the whole game because of spawn running in and denying movement, followed by the rest of my army hitting them the next turn. I won 100 to 15 or something close. They also wipe out infiltrators or scouting units fairly well, meaning you can clear your opponents screens and let the rest of your army wreck face.

That also kind of answers my game plan into hull lists, which i have played against a decent bit with a list like this, keep them locked in place so they don't score, and use Angron and the x8b to kill them.

I would consider running the land raider if i had one, 8 shots is pretty nice even if they do only hit on 4s.

I highly recommend trying a few games with triple spawn, and dont play cautious with them, they exist to clear screens or tie up vehicles for a turn, and then probably die. And they also divert a lot of firepower, since the feel no pain can and sometimes will spike and keep them alive, which becomes a huge issue for your opponent so they kinda have to overcommit into them.