A close friend was employed by a well-known manufactured home builder here, good reputation and happy customers. However, he had to visit every single home for remedial work ... the homes take some abuse in the move and there's nothing you can do about it. Drywall cracks, windows and doors that cracked or needed adjustment, that kind of thing. Nothing serious but make sure you have the warranty and talk to some owners.
There is another firm here in the city that builds all their homes indoors in a huge hangar-sized facility. These home have an excellent reputation, no remedial work required most of the time ... they just drop in with a crane, second or subsequent floors are separate and added to the established main floor with the crane. Tradesmen I know have nothing but good things to say about them.
Both are less expensive than building on-site, trades are there and just move from building to building, plumbers, electricians, whatever. With the indoor builder there is also no rain, no weather issues, temperatures of materials are all stable, etc.
So costs are lower, no travel to sites, less time wasted going from building to building, small problems don't hold the job up as there is always other work for the trades and they can come back to yours in one or five days when the problem is sorted out (wrong parts, etc). They are also 100% custom build and without extra costs associated with that.
The big thing is to vet your builder heavily. That makes all the difference. Next would be your building mover/ crane operator, as they can only break things. If all checks out it's a slightly less expensive method of home building, but beware because issues can quickly get out of hand if the builder and mover don't have a good reputation with other owners. Do your due diligence and you should be fine.