“So far” I’ve found they are reusable, both header and collector gaskets “If” you only loosen the bolts just enough so you can wiggle (parallel, sliding action...no pulling) the headers or exhaust pipes to break them free from any sticking and then remove the bolts.
Definitely can’t just pull the bolts and pull the headers/exhaust as the Remflex can stick and then they aren’t usable like you say.