Have they ever said what exactly happens if someone strong in the dark side uses a "natural" kyber lightsaber? Does it start to run hot and short out? Does it start wobbling the handle? Will it try to roll away when it's put down? It looks like whatever the problem is, it doesn't kick in immediately, based on Vader using his original saber for a day or so without a problem (or much longer, depending on if you think the crystal might've gotten suspicious after the occasional massacre and murder before he "officially" turned).