I would disagree on that. Quitting smoking is a fairly simple thing, but it can also be a very hard thing to do. Such is the nature of addiction. So as such I find it very honest to say it is simple but hard, and focus on what makes it hard.

Somebodies reaction to a hard thing may be complex, but that is more about them than the thing. Although as you note, there is a risk of oversimplifying (e.g. your example of possible implications of gut biome, but that doesn't really shift the fundamentals).

The reason I think this distinction is a good one to make is the problem with the converse. If something being hard implies it is complex, it will lead often to people generating extraneous complexity around it which makes it actually more difficult for the people involved, and can lead to very convoluted thinking and errors in that thinking. You need only look at the massive industries built around things like weight loss and quitting smoking to see examples of this (not that everything in those industries is nonsense, but they do contain a lot of profitable nonsense).

This had nothing to do with bubbles. The risk is that instead of supporting people in doing a hard thing, we raise false hope of making it easy.