FF lens on apsc, good or bad?

No, your thinking is correct. With a 1.5 crop factor, you'll only capture 1/(1.5)^2 of the maximum possible resolution of the lens. So if the effective resolution of the lens on FF would be 20MP, you'll come out slightly short on 16MP. The numbers I give are entirely hypothetical as you still haven't said what you're actually considering purchasing.

Looking at the problem the other way around, letting your sensor slightly exceed the capabilities of your lens is still preferable over buying a camera with a lower resolution sensor that matches your lens, as you can always claw back a small amount of extra sharpness in post processing.