I've used them for mostly dedicated tasks, at least the RPi3 and older. I've used the RPi3 as CUPS servers at a couple of sites, for a few printers. Been running for many years now 24/7 with no issues. As I could buy those SBCs for the original low price and the installation was a total no-brainer, I would never consider using any kind of mini PC for that.

I have a couple of RPi4 with 8GB and 4GB RAM respectively, these I have been using as kind-of general computers (they're running off SSDs instead of SD cards). I've had no reason so far to replace them with anything Intel/AMD. On the other hand they can't replace my laptop computer - though I wish they could, as I use the laptop computer with an external display and external keyboard 100% of the time, so its form factor is just in the way. But there's way too little RAM on the SBCs. It's bad enough on the laptop computer, with its measly 16GB.

I built a nice little cyberdeck around an RPi 5 but it's turned out to be very disappointing. I was counting on classic X11's virtual display stuff to enable a 1080x480 screen to be usable with panning (virtual 720p or something, just a cool vertical pan). Problem is, the X11 support sucks, and so there's almost no 2D acceleration, so this simple thing that used to work great on a 486 with an ATI SVGA doesn't work very well at all on a machine a thousand times faster. Wayland has of course no support for a feature like this one, so I'm stuck with a screen too narrow to use, and performance for everything else that's pretty sub-par.