One theory behind ARM's power efficiency is that you don't have to waste power and area converting x86 into uops. But this is a constant overhead. It matters a lot on 5/10W TDP mobile processors, but matters much less on 45W TDP laptop processors.
No. They are different use cases. Personally I prefer the 45W chips with higher base clocks at the expense of battery life. 15W is unsuitable for some workloads, though the latest 15W "U" chips from AMD are making some waves there.
I spent a week recently working from a Pinebook Pro after spilling water on my primary laptop; my workflow was already fairly SSH-centric, but I had fairly few problems with software compatibility that weren't fixed by binfmt_misc + qemu. (That week was spent mostly developing in OCaml and Lisp; I suspect if I were doing C++ or Haskell, I would've had a harder time.)