They must be unique in a LAN segment. And only the lower 3 bytes in a MAC are “unique” as the upper 3 are the vendor ID and relatively fixed.

In practice people put fewer than 256 devices on networks (class C), so they have less than 1/65536 possibility of complete failure. And far less because they have a mix of OUIs.

But yeah, if you put a few hundred or thousand security cameras or other device from a single vendor, all on the same network, conflicts are certainly possible.

MAC conflicts are also a bit nasty to troubleshoot, and less obvious than IP conflicts.