I was doing cybersecurity for a few years before I moved to gamedev, and I vaguely remember that at least the older versions of GUID were definitely not safe, and could be "easily" guessed.

I had to look it up, in case anyone's interrested, and from a quick glance to the GUID RFC, it depends on the version used, but if I'm reading it right, 6 bits out of the 128 are used for version identification, and then based on the version it's some kind of timestamp, either from UTC time or some kind of a name-space (I didn't really read through the details), and then a clock sequence, which make it a lot more guessable. I wonder how different would the odds be for different versions of the UUID, but I'm too tired to actually understand the spec enough to be able to tell.

However, for GUID version 4, both the timestamp and clock sequence should instead be a randomly generated number, which would give you 122 bits of entropy. It of course depends on the implementation and what kind of random generator was used when generating it, but I'd say it may be good enough for some uses.

The spec also says that you specifically should not use it for auth tokens and the like, so there's that.