Guest networks are configured to disallow intranet connectivity, so the "security" risk would be from the device having access to the internet, on a guest network IP. But a guest network exists for the purpose of giving untrusted devices access to the internet, without compromising the security of the intranet. My local library, the local airport, Home Depot, the movie theater, etc all have public wifi, yet nobody would claim they were directly compromised. I understand the connection could be used for receiving and sending packets over the internet, like a DDoS (which would trigger rate limits, even at Home Depot), but framing a guest network, giving an untrusted device access to the internet, as some security issue seems very odd. The isolation is the mitigation.