Many hyperscalar server setups use POWER8/POWER9 CPUs. 4 logical processes per core (and 8 with the upcoming 15-core POWER10 configurations) are pretty useful when measuring perf-per-watt.

The Talos is currently the only fully libre computer available for high-perf computing, and it uses POWER9 CPUs. If you want a fully free CPU, your choices are either very dated CPUs or POWER.

Many distros (inc. Debian, and most source-based ones) support ppc64/POWER officially quite well and go out of their way to ensure a high degree of portability.