I'd expect them to be significantly better than the competition in that area given the large part of the CPU that is a dedicated coprocessor specifically for crypto (called CPACF). There is much less area given to crypto on x86_64.
Of course we are talking about encryption here. TLS and AES etc etc. Not Bitcoin mining, which would indeed not be very cost effective.
They have a lot of cryptographic functionality built directly into hardware, and IBM is touting quantum resistant cryptography as one of the key features. You won't mine Bitcoin on one, but, if you are concerned a bad actor could get a memory or disk dump of your system and store it until quantum computers become practical, IBM says they have your back.
Of course we are talking about encryption here. TLS and AES etc etc. Not Bitcoin mining, which would indeed not be very cost effective.