It cannot be overstated how nice it is to move away from erb and into pure ruby. Private methods as partials is a game changer for refactoring your UI.
Not a fan of the rails erb helper methods though, so maybe it doesn't count. I almost exclusively use it outside of rails now
It's got a nice path for incremental adoption, so there is no need to go all-in unless you want to. Personally my Rails projects are all 95%+ ruby now.
Another nice knock off effect is that the LSP and integration with my IDE (neovim) is much nicer with Phlex components than it is with ERB.
I also think personally I don't like "magic" spaces like ERB where its hard for me to follow the code path. How does ERB manage multiline blocks with html in them? Hard to understand, harder to explain. To me it feels like there is less to worry about in Phlex where I'm thinking more in Ruby, less in the specifics of the view context.
To create them declaratively (not needing to call them in a set order) you just create a module that defines `before_template` and calls `vanish`: https://www.phlex.fun/miscellaneous/v2-upgrade.html#removed-...