How secure is WinVNC ?

How secure would it be to run WinVNC server service on PCs behind NAPTed routers?
Could it be hacked from Internet via ports 5800/5900 it runs on?
We do port mapping at router side for internal services (SMTP, HTTP, etc.), and , of course, 5800/5900 are out of the picture, but concern still remains. Share your experience or know-how. Greatly appreciated.
Thx