I found using partials between projects to be a headache. I wanted a way to standardize my UI for accessibility across all projects and not have to re-implement the same standards over and over. It's nice to be able to package the UI up into a gem and keep it maintained everywhere.

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.