Personally I think htmx is an interesting strategy because it unifies the rendering tech on the server (you still gotta be careful to properly scope your incremental changes).

"Advanced forms" come up all the time, and are usually when you have even a bit of non-trivial business logic. For example "if people pick this set of options, show this other option" (but you want to avoid having a form wizard, cuz then you're having to track state across multiple page loads). There's also stuff like in business applications, previewing calculations and the like.

"A few JS event handlers" can handle a lot of tiny things, but many B2B SaaS have a handful of pages with a loooot of these things, and at the end of the day you could have your hacks, or you could try to be principled about loading it.

Though it's also about just finding the right mix, there are people who can architecture their stuff "correctly", completely SSR + some JS flavoring. But it requires having people who are really good at backend and frontend figuring out the right pieces and putting it together. That diligence is hard!