Synths are machines built to serve, with a home and a purpose.
Humans are kidnapped, abused, dehumanized, and sold to other people.
Comparing the two is an insult to people who have been affected by slavery.
Synths are treated relatively well, compared to slaves in FO3. It's clean and safe in the Institute, and people recognize they have human needs. On the surface, the Railroad would die to protect synths. But they don't do a thing for human slaves.