It has ways to indicate "things are in flight" but for distant deployments you probably still need to write JS to manage some in-page elements for reasonable feeling UX.
This isn't hard, it has support for interop with plain JS or existing libraries and a somewhat limited built in system for basic show/hide/set/remove attributes (somehow lacks a "toggle_class" and "toggle_attribute" function though so you can write those yourself...). You can dispatch any custom JS event from it to cover .. anything you need.
If you're exclusively deploying to on-site/one-site/geographically-constrained systems, then you actually skip that issue...
You also have to re-think about how write code as Elixir is immutable, though I see that as a bonus.
Phoenix (and by extension LiveView) are actually a lot smaller than you might think coming from Rails or the like, mostly leveraging existing tools.
Elixir/Erlang/BEAM make building systems incredibly pleasurable. It ships with many tools to make and connect complicated things together, in a reliable way. Depending on what you make you may find it very liberating after working with Node.