As a retired homebuilder, I'm always amazed at how well this industry has managed to confuse the public, and has done so strictly for their own gain.
My last house was a State Code modular home that was built very similar to a site built home. The exception being that all systems and structure were built to a higher standard than is typical for this market, and it came with an unusually steep roof pitch, and many high end finishes and products installed such as central vac, a huge solid cherry kitchen, Anderson windows and doors, real plywood subfloor, 5/8" plywood roof deck, etc.....Now I could of taken the same plan to another regional manufacturer, and downgraded everything to the point that it would of cost me 20% less and been only slightly more durable than a mobile home.
Last year the place sold for full appraisal, without any deduction for the fact that it was a modular, and did so without a realtor, and before it was even officially offered for sale.
OTOH, the industry is quite deceptive in describing their products. Single and double side trailers were, are, and always will be pretty sketchy products. They are often sold with a VIN # and title from the state DMV, and the have two things in common. First, they are built to federal HUD standards, not to building codes like the IRC, ICC, BOCA, etc.... These HUD codes are the bare minimum to erect a dwelling that is somewhat safe and durable enough to just get the job done. The other commonality it that they are built on a permanent frame, and the frame is an integral part of the unit. A state code modular is built in the factory and lifted onto a "carrier" which is used to deliver it to the site. At that point a crane removes the sections and places them on the foundation.
The industry profits from confusion in all of this by never using the term "trailer" or "double-wide" since they both have negative connotations. Instead, they market trailers as "HUD code modular" or manufactured homes.
Bottom line? It is quite possible to buy a really top notch modular, and end up with a product that is superior to a typical site built tract house. I've built countless new site built homes, fixed and remodeled a few trailers, and housed my family in a great modular for a decade, so I have been involved with every end of this spectrum. There is a lot of prejudice and misunderstanding involving anything that is manufactured, and the level of ignorance and lack of acceptance when it comes to state code products is very regional. PA. is the center of the state code industry in the northeast. A state code home here doesn't raise an eyebrow from buyers or banks. This is quite different that places like Texas, where I have spoke to others in the building business who are firm in their belief than anything not built on site is automatically crap, and will depreciate. It's kind of sad, since the site built industry is about a half century behind modern manufacturing, and the availability of tradespeople who are not illegals, and reliable and not drunks or druggies, is becoming a major problem in many areas.