If it's about the predictability of a given vehicle's projected maintenance requirements, use unreliability.
If it's about the excessive technology leading to unreliability, maybe discuss complexity.
And if it's about a quality record of the manufacturer which changes markedly over time, discuss variability or inconsistency.
But I'd expect instability in discussion of vehicle-related matters to refer to road-handling, steering, or the like. If that's the case, then discuss uneven handling. And if it's not the case, then avoid confusion (or unjustified criticism) that might be caused.
If you've got to argue for or justify the language in a mass publication, it's poor language.
A "stable marriage", for example, is conceptually stable in the same way a plane or car is physically stable.
To then take that analogous usage form and re-apply it back to where the actual, original meaning makes more sense, is indeed confusing.
> Electric vehicle drive systems, or powertrains, are not the main source of problems, Fisher said. The issues reported by owners are with other features.
> Manufacturers "are using EVs as a technological test bed," Fisher said. "Those are the things that go wrong."
That's instability leading to poor reliability.