Powershell's completion is worthless for the most common use case of saving keystrokes. Since it fills in the entire remaining command name and cycles through the possibilities in alphabetical order instead of completing just what's unambiguous and presenting a list of the possibilities from there, it doesn't help with typing out common prefixes and if you find yourself cycling through an unreasonably large number of possibilities you have to backspace get rid of the unwanted completion (often very long, given the naming conventions) before you can refine your search.

They took a code completion technique that works alright for an IDE and put it on the command line, losing some key usability in the process when they could have just implemented the paradigm that has been standard in the Unix world for decades.

Yes, it does a great job of identifying what the completion possibilities are in almost every context. That's good enough for an IDE, but only half the job when you're making an interactive command line shell.

As for other readline features: it's really annoying to only partially implement a well-known set of keyboard shortcuts.