Unless they’re sending you into a mountain when given a heading assignment, that would be one of the only few times that I would consider responding with unable. The controllers have passed the required practicals to operate a radar frequency. If you’re using unable you’re doing it wrong. You may not know what the plan is, but the controller knows what the plan is. After all, you requested a service from them, so you wanted the vectors to airport you are intending to land at.