Z has plenty of custom hardware - I think it's fair to say it's predominantly custom - the branch predictor would have to be pretty different, and of course power doesn't have a BCD arithmetic unit.
Actually, it does have IEEE decimal floating-point, as does z/Architecture. z/Architecture has decimal fixed point, but, these days, it might just trap to millicode doing tricks such as excess-6 for carry propagation. (And the PowerPC processors in at least some AS/400 machines added some instructions to assist BCD arithmetic.)
Anyway, I'll argue that they're spiritually and economically related, and there's more than a passing family resemblance. Kind of like power and modern ("advanced server") iSeries,
There is no iSeries any more, there's just the IBM Power Systems, which are the successors to both RS/6000^WpSeries^WSystem p and to AS/400^WiSeries^WSystem i; they can run both AIX and IBM i.
Meanwhile, channel controllers aren't as dumb as they look. A little wikipedia action here (I know, citing wikipedia, but it's monday and I'm still tired): https://en.wikipedia.org/wiki/... . Turns out the little dickens can do a decent amount of work on its own. I think the wikipedia entry is showing its age... seems like IBM's done a lot more work since this.
Yes, but they're still I/O boxes, not general-purpose computers (well, they might be implemented with z/Architecture or Power ISA processors, but what's exposed to the OS or application programmer is just the ability to run limited channel programs). The z/Architecture Principles of Operation says in "Execution of I/O Operations", in chapter 15 "Basic I/O Functions":
For subchannels operating in command mode, the channel subsystem can execute seven types of commands: write, read, read backward, control, sense, sense ID, and transfer in channel. Each command, except transfer in channel, initiates a corresponding I/O operation.
For subchannels operating in transport mode, the channel subsystem can transport six types of com- mands for execution: write, read, control, sense, sense ID, and interrogate. Each command initiates a corresponding device operation.