Generators (either variety) in JavaScript are flawed in their implementation in that they only went halfway with it. They also needed to make all primitive objects iterable as well, and/or they should have introduced iterable helpers for all data structures. The fact that you can't use an object in a `for` loop without implementing Symbol method(s) is a glaring hole in their every day utility. It makes it non-obvious to reach for in many cases

The other to me is some of the semantics of generators are not well thought out, for instance, you have to call `.next()` _twice_ in order to get the first `yield` value, and how arguments to `.next()` should be used correctly is opaque. This combined with the fact `yield` doesn't follow the same lexcial scope as `this` does (IE, you can't have an arrow function yield if it was enclosed by a generator, unlike `this` which can be used inside an arrow function as reference to its enclosing scope. This would make generators far more useful IMO).

Combine all this with the fact that most frameworks don't support generators natively for things like components (but they are starting to accept Promises / async functions) you end up with a relatively niche feature

Generators are one of those things that pop up more often in library code than in application code, I've found.