They don't have to be serialized unless you sell them (for a living, and have a manufacturer's license, hobbyists kinda just can't sell guns they make.) Or if you take them into a gun shop to be worked on, then they have to serialize it.
There's like 6 states that have harsher laws than federal on them, I think in those states they have to be serialized, I don't think they can stop you from making one though, but all other laws apply too so it kinda ends up being a "you can but really not" type thing.
But even then, even in states where it has to be serialized, the only way they'd ever know someone had an unserialized one is if someone was caught in a crime with one or defended themselves with one, and imo crime is already illegal but defense being criminalized because of the lack of a few letters stamped on is dumb, and it really just gives the police another excuse to "crack down" on minorities (as if they need another reason for what seems to be their occupational pastime.)