At the end of the day, both bash/zsh are "dumb" shells, they don't have the introspection capabilities Powershell has regarding commands they execute. That's both a benefit (they're flexible and you can throw anything at them) but also a disadvantage (they can't really help you much out of the box when you want to run a command; they are only able to help you regarding things they find out from the kernel, such as files, but not arbitrary info from the commands themselves).
The reasons that bash/zsh are faster and better supported boil down to history: tens of years of production usage by millions of people. As a result there's this cottage industry of tips and tricks, autocompletion scripts, etc. that makes bash and zsh usable.
I'm willing to bet that with a similar amount of man-hours poured into its ecosystem Powershell would beat both of them.
But as I said, we live in the real world and both bash/zsh are faster and a bit more robust than Powershell.
I'm not sure what you mean with this.
If you're referring to feature set and robustness with typing in scripting, I might agree with you.
I have however a much tougher time agreeing if you also think that powershell could come even close to bash/zsh when working from the shell, in both speed of use (and as you said) support.