x86-64 is a CISC architecture

In many cases it's actually RISC under the hood and uses an interpreter to translate the CISC commands and run them in the most optimal manner on the silicon

ARM and RISC-V absolutely scale up to multi-hundred watt server CPUs quite easily. Just look at the Ampere systems you can rent from various VPSes for example

The big benefit that ARM and RISC-V have is they have no established backwards compatibility to keep carrying technical debt forwards. ARM versions their instruction sets and software has to be released for given versions of ARM cores, and RISC-V is simply too new to have any significant technical debt on the instruction set side.

Atom cores were notable for focusing the architecture on some instructions then other instructions would be a slog to execute, so they were really good at certain things and for desktop use (especially in the extremely budget machines they got shoved into) they were painful. Much like how eCores are now. They're very carefully architected for power efficiency, and do their jobs extremely well, but an all eCore CPU is a slog for desktop use in many cases