They're also pretty good for special use vehicles, a Mitsubishi Outlander PHEV is pretty much the best EV-ish vehicle if you need to tow a bigger load for long distances without shelling out 100k€+. And you can still drive on full electric around the town cheaply.
Considering they are unused most of the time, I'd expect very infrequent maintenance, and even some parts switching from X years to lifetime.
Edit: a short Google search [1] shows that for actual cost, plug-in hybrids have the same cost per mile than EVs, which is half the cost of ICE cars. It makes sense to me because batteries are a lot smaller/cheaper, the ICE doesn't get used all that much, and there is regenerative braking.
[1] https://www.greencarreports.com/news/1129728_plug-in-hybrids...
The Prius engine is a good gateway drug to full EV ownership, you'll learn that the amount you press the accelerator doesn't correlate with the engine RPM at all. The car picks whatever it thinks is efficient to move the car, any extra is stored into the EV system.