Swallow Whole used to be my favorite mechanic with large creatures like dragons, hydras, or what have you.
I mean, it's both narratively and mechanically a powerful ability. Usually the creature gets a bite attack with Improved Grab, so they get a free grapple if they hit. Then the next turn if they have a grappled creature in their mouth, they swallow them. From there it's usually continuous acid / bludgeoning damage, but the swallowed creature gets a chance to fight their way out or get rescued.
Being grappled was already somewhat debilitating, but you could usually ignore it. But when you get swallowed it immediately raises the stakes. It also means you can't ignore getting grappled because it can become so much worse so quickly.
Then one day one of my players noticed I was using the ability a lot and accused me of having a vore fetish. Now I don't really use Swallow Whole at all
