That's true, the shotgun shoots peanuts in comparison to even just the Strikes ALONE, nevermind the crazy strong grapple.
The best idea would be to have the grapple not be a free kill unless the target is injured or has low hp. I can see the devs wanted the grapple to be more of a Takedown rather than a traditional grapple like in fighting games (Street Fighter, Mortal Kombat) but it would work better if it didn't have this many benefits.
Removing invincibility and having it do like 50/80% of the target's HP or something like that would be better, Maybe have it instantly kill only if the target doesn't notice you, but some characters are always visible, so that's a problem, but it would make the stealth aspect of the game more important that way.
Basically, have the grapple work as a takedown in stealth/enemies unaware and as a finisher in melee combat when the enemies are aware of you. And yea, the invincibility is way too much. Having to watch your teammate get grappled or yourself and not be able to help or get help kinda kills the cooperative aspect.