Most the the factory constructed builds I have seen achieve better insulation and air sealing than traditional platform constructed homes.

There are lots of levels of factory build. SIPs (structurally insulated panels) built in a factory and assembled onsite with all finishes done traditionally. Factory cut lumbar exactly to measure and built like lego onsite. All the way up to entire buildings built offsite in chunks that can be transported by truck with gaskets to connect the sections. Connect the plumbing and electrical and move in the next day.