I presume you are referring to UUIDv4/1, which has 122 random bits. It is a common misconception that this is insecure. In reality, the UUIDv4/1 is just a way to encode 122 random bits, which is more than enough for most needs. The property of being practically unguessable comes from the random generator, not from the encoding.

Secondly, whether or not being predictable is a problem depends on the use case. The keys used on this forum are predictable and it is not an issue.

I’m pretty sure you mean ‘stable’ not predictable here re:MACs?

The issue with using Mac addresses is it essentially leaked tracking information on the machine being used to generate the uuid - you could associate every ID it ever generated back to that same machine, correlate it to things like open WiFi AP logs, whatever. Which is pretty scary sometimes.

Many OS’s now randomly generate a new MAC on every disconnect/connect for this reason, to at least give some privacy.