Plastic grommets

The same as the HP of a Rolls Royce automobile engine.
"Adequate"

Why?

Buy and test some to find out.

If you are asking for the purposes of design, I am quite sure the manufacturers of these components have engineering data available for each specific design, of which there are many. Ask them.

They're normally used for securing trim panels, interior components, etc., for which the consequences of a failure are non-catastrophic. Cheap, only requires a simple hole in the mating parts (as opposed to threads), very quick to assemble on the assembly line, and if you pick the right ones, they can be removed non-destructively and re-used. Some of them require access to the backside to push the pin back out. I'm OK with the reusable ones. I hate the ones that resist non-destructive removal.