An inconsistent voltage is a bit of a deal breaker if it’s a significant swing. For example, AA batteries are supposed to be 1.5v. Typically you can measure them at 1.7v when they’re brand new, and that’s fine. The voltage will drop as it’s used and devices tend to stop functioning when the battery reaches about 1.3v.

That’s a fairly narrow range of voltage required to keep the device working correctly.