> there's no real way to track whether a US citizen overstays more in some parts of the Schengen area than others.

Doesn't matter, it's 90 days applied to the entire Schengen area, regardless of which member state(s) you visited.

Yes, but the person I was responding to found it relevant whether Americans are more likely to overstay in particular EU countries such as the ones on which the US imposes visa requirements. The only possible tracking (for short-term entries) is at the Schengen level, as you say; the US does not require visas from most Schengen nationals. But the US does require ESTA+VWP from them if they don't seek a visa, and this is basically analogous to ETIAS.

But (for now), there isn’t always a passport-control between Schengen and non-Schengen zones.

E.g. your time in Monaco, Andorra or Ireland.

Dunno if your time in Ireland is kept track of or not if you fly to/from it.

Self-replying to note that one of the EU member states on which the US imposes visa requirements, Poland, is already in Schengen. (Edit window has passed.)