"Dropdowns are toggleable, contextual overlays for displaying lists of links and more. They’re made interactive with the included Bootstrap dropdown JavaScript plugin"
- don't follow platform conventions (don't trigger the actual system dropdowns with actual expected behavior)
- get cutoff by browser chrome when page is zoomed in (because you can't control that behavior from JS)
- default implementation's touch targets are too small
And that's for a framework which has had over a decade to make this work.
This is not to throw undue shade: they've done a very good job. But that's about as far as you can get.