Can opening ports for hosting servers be dangerous?
Having the ports open for the server is not dangerous in itself. The server exploits that pop up every once in awhile are more dangerous.
Server exploits? What do you mean?
About exploits n stuff: A while back there was a very serious exploit in srcds that let anyone upload and download (most) files from your server, letting people completely take over servers etc, but I believe that’s been fixed, and stuff like that rarely pops up.
Opening ports is only dangerous if the servers that listen to them are insecure/not designed to be externally usable/have security flaws. Normally this isn’t an issue with most server software (especially with big stuff like IIS/Apache) however it’s a good idea to have the firewall turned on to its most aggressive settings (e.g. all blocked except what you need) anyways, the fewer ports someone is able to connect to at all, the fewer they might be able to exploit.