Yes, many. My completion rate is probably <30%.
Things like part availability are rarely the factor - mostly I find its just indecision. I tend to brainstorm a problem - which is usually only a tiny part of the total project - and come up with 4 - 6 solutions, discount a couple for cost purposes, then can't decide which of the remaining ones to actually implement.
I suspect a few decades ago with much less availability (and choice) of cheap programmable solutions it was easier since there was only a couple of ways of doing things with a large difference in cost/hassle, so decisions were much easier to make. These days? I have a choice of tens of different micros, from several manufacturers, different displays, encoder? or buttons?
And then theres feature creep - what starts as a quick test setup to do a frequency sweep for audio filters turned into a full fledged, programmable system - that I have never really used.
Edit: I forgot to mention something that is much more a personality "me" problem than the technology - I really enjoy getting an inital idea and brainstorming, making frantic notes of all the ideas I have for the project, extra features, clever tricks/use of logic or micro's peripherals, then I start testing and prototyping... but then theres the boring part of just building it. Then I get distracted by yet another idea, and start the creative frenzy with that - whilst the first one was only 10% built. Rinse, repeat. I have to actually force myself to finish building things by dedicating an entire day to just that and it seems to be working well, going through my backlog.