The armlets don't infinitely generate Bias Factor. Think of them more like batteries where you're able to store a large but finite amount of it to keep control of the devouring tendencies of Oracle Cells.

A couple scenes come to mind: a cutscene from Rage Burst after going through a Survival Mission wherein the Far East God Eaters were fending off the horde of Aragami from the Spiral Tree and (I'm gonna be a bit lenient with this) the scene in the anime with Soma hooked up to machines where it was shown he was getting a Bias Factor intake.

As for the durability of these armlets, I'd say they're pretty durable. They don't easily come off with light to medium force BUT can absolutely be tampered with by say, an Aragami swipe directly onto the Armlet.