Comparison to CryptoNight
In XMRig POWER Performance Discussion , TimothyPearson says that CryptoNight v8 damaged POWER's competitiveness against x86 by emphasizing integer square root and integer division. Based on looking at RandomX Integer Operations , JeremyRand's impression is that neither of those operations is performance-critical in RandomX (which replaced CryptoNight v8 in Monero in the hardfork of Nov 30, 2019). So POWER is likely to be competitive against x86 for Monero mining again once the RandomX JIT is ported to POWER.
Unfortunately, RandomX cannot use POWER9's NX-AES accelerator, because NX-AES is not currently exposed to userspace in Linux.