Do you have any idea what insecure could mean or the downside of using it, if I was to open a wallet with coins using this would they be at risk of being hacked?
The UPNP library included with certain versions of Bitcoin Core were susceptible to buffer overflow. This potentially means that an adversary executing an attack against you could take advantage of that to do a remote code execution.
You just have to disable UPNP in the settings, turn your internet connection off and it should be fine. It's not that easy to exploit.
