Although it (DIY hardware synth projects) might seem limited compared to software synths, it also has dimensions that a lot of virtual instruments lack. I’ll call these qualities depth and exclusivity.
Depth: When you turn a pot on this hardware, you’re really in control. You may argue there’s a lot of controllers out there, but compared to the 1024-level resolution of the Arduino, standard MIDI Control Changes turn out to be bogus for smoothly controlling parameters. Just playing with the default FM patch of the Arduino piano makes it obvious.
Exclusivity: When you play with hardware like the Arduino piano, it’s the only thing you do. You can’t fire Google or start reading your mail, and I think it’s really valuable. Every music “tool” should be able to immerse you enough so that the rest of the world doesn’t exist. All computer-based synths have failed to do that for me.