Not necessarily. It completely depends on the particular app and how they’ve built it.

You’ll find it increasingly common that people are using cross platform frameworks that allow them to effectively build the same app for web, desktop, and mobile so there really isn’t much of a difference between any.

They get less attention. Are usually there to supplement the app so the bar is lower.

What if there is no corresponding native app? Are web sites more buggy than competing native apps?

Is this building more prone to falling apart than that building? Depends how it was built. In other words, no.