1) Rats nest of non-declarative JavaScript.

2) Rats nest of JavaScript callbacks.

3) Overlapping stylesheets with !important everywhere.

4) Elements used for style not their semantic purpose (<b>, <strong>)

5) Subtle and not-so-subtle browser compatibility issues.

You can't reuse pieces of UI functionality (a bundle of specific html, css and js) unless did copypasta which is obviously awful. Sharing business logic across different areas of an application becomes much harder and you can't non-trivially write and run tests across areas of your application because you are just using html,css and js. And the big monster: state. Sharing state safely across different areas of an application becomes much harder. You end up writing your own micro framework trying to make all the above work. At that point you might as well use as existing battle-tested one

Using them was fine. but developing them, no. It's way more work just managing event handlers for a start.

mainly the CSS