Also be aware that an exposed port that has an application responding to requests can give information that might reveal weaknesses (for example old versions with available exploits).

I know Minecraft was very exploitable earlier, I guess with that specific version an attacker would still be able to get access to your machine in some cases.

So port forwarding is like unlocking a door. As long as the stuff inside the door knows how to handle unwanted guests then no problem. But the challenge, as others have mentioned, is to make sure you actually know everything is secure.