Beyond the obvious differences in knobs, buttons, and displays - the three systems are basically in the same ballpark. For the most part, the memory size and processing speed, audio I/O, and MIDI I/O are close enough that you can achieve same range of musical aims with any of them.
They are all running Linux, they all use standard Linux facilities for accessing audio and MIDI. They can all run the same music software: SuperCollider, Pd, MODEP, Tidal, Orac, Orca, etc…
The significant differences are really in what has already been set up and configured. You see people doing different things with these devices, because the pre-packaged environment is aimed in somewhat different directions.
There is great value in not getting side tracked in the tool building itself - so if you want to get on with something you see that norns does… a norns shield or fates board swapped out for your Pisound (or just get another Rpi) is going to be the easiest and quickest way to do those things.
You can get the whole norns software stack running on your Pisound box - and get it to use the Pisound hardware for audio and MIDI… But you’ll also need to attache buttons, encoders, and the display… It is a large configuration effort. Remember that norns is not just the hardware, it is a configured Linux image ready to go.
On the other hand, if you’re interesting in building SuperCollider or Pd instruments then you can definitely do that on your Pisound based system. Personally, I run my SuperCollider on my Pisound, as well as C++ based MIDI processing software I wrote.