Whats the definition of "serious" here? Is there a standard measure of tolerance or material compatability that it can be rated against to see if it would work for a specific need?
I'd probably argue that anything worse than 0.5mm (~20 thou) will start giving you headaches if your making parts that are supposed to interact with each other. That said, if you're making basically oneoffs and are willing to handfit, that tolerance is probably tolerable (ha!).
"Serious" just means the ability to cut more useful materials, like wood or steel.