I see two reasons for building a gun from parts. One is saving money, because the 11% excise tax has not been collected, and the other is to avoid a trace. The Pittman-Robertson funds from excise tax and licenses have done a lot for the shooting sports, including ranges and habitat projects, so I am only cheating myself by not paying this tax. The other would be to eliminate the tracing of the firearm, and I am more likely to benefit from a trace if it is stolen than to suffer because the gun was used in a crime.
I can't imagine owning a gun made by someone I cannot identify, so I don't have to worry about customer service or warranties. No, unfinished guns have no attraction for me. Those who think they may become more valuable because they will become contraband should have a backup plan, because we do not know what the future holds.