The Phoenix framework was created by Chris McCord. He was a long time Rails developer who tried to create a LiveView equivalent in Ruby/Rails but concluded that the concurrency capabilities just resulted in too many edge cases that didn't work well. That process led to him working more with Erlang and Elixir, then creating Phoenix.
I have never used Stimulus or Stimulus Reflex, but based on his output Chris is a smart guy and an excellent developer who's opinion I therefore give some weight to.
No doubt, but there are some pretty awesome wizards working on StimulusReflex (and CableReady which it sits atop) as well, so I think it mostly depends on if you're keen to program in Elixr or Ruby.
I have never used Stimulus or Stimulus Reflex, but based on his output Chris is a smart guy and an excellent developer who's opinion I therefore give some weight to.