I think it's a combination of both!
In my anecdotal examples, ALL my bean plants have mealy bugs, yes? But if I forget to water a single one and it becomes stressed, the little pests swarm on that individual. But they are still present on every other plant even though those are in tip top shape! (Oh, I hate scale insects!)
The weakened plants probably make an easy target but I'm sure the healthy ones are better. But the more good they are, there must be a threshold for the bug because they have higher toxic defence?

I wish we could just interview an insect