I agree with David. Mostly it is a question of how usable things are, I suppose any computer can do just about anything with the right software. And the backwards compatibility we take for granted in the RISC OS world ensures that just about anything that runs on Iyonix can also run on a RPC. But if things are too slow the experience can be painful and you tend to give up.
Things an RPC can't do (very well, copmpared to an Iyonix or modern PC) include
Providing a workable display at 2048x1536 pixels in 16M colours
High quality photo printing using a range of modern photo printers in a reasonable amount of time.
Access USB devices (at least without an add-on).
100 BaseT / 1000 BaseT Ethernet.
Rapid rendering of large images at high resolution (eg for digital photography).
Faster rendering of web pages
I'm sure there are lots more. But my point is that you probably wouldn't want to use your PC in preference to your RPC if it was a 386 running Windows 3.1, ie a computer of the same vintage. I suspect you prefer you PC for many things because it is a more modern design, with a faster processor and better software. This has very little to do with the operating system, and is mostly to do with the advance of hardware technology. The software argument doesn't really apply to RISC OS, since unlike Windows apps most modern applications will run on old versions of the OS.
I find that I can do many things at least as quickly, if not more so on the Iyonix compared to my PC (2GHz Pentium running Win 2K). I don't think that that would be the case with an SARPC.