That's a pretty sweeping generality, that just doesn't apply to most of the US real estate market.
Lenders cannot charge higher interest rates simply because a home is manufactured. That's against the law. Local building codes,construction standards and product demand determine the quality of product acceptable in a market.
The risk factors are the same as for any home. Neighborhood, environment, property risk, quality of construction, etc. If you're buying a sardine can and parking it on leased land, don't expect a mortgage loan. You are basically buying a car. These products cannot be underwritten with conforming loans. Sorry to say, but I have seen homes sold in the South that would never be permitted for human habitation in CA.
On the other hand, the quality of constuction in manufactured homes has improved dramatically in the last 10 years. Appreciation on new homes is nearly at par with stick-built in CA. Time will tell how the newer homes eventually compare in equity to the stick-built ones.
Then Michigan is an anomaly. Mobile homes are selling but they don't appreciate. Its taken this long for prefab/kit houses to catch on, and even now they don't sell as well or at as high a price as a stick-build.
My problem isn't with the trailers, per se, its with the way they are taxed (or NOT!) in Michigan. One of the reasons communities fight new parks so hard is they're only paying $36 a year in state and local taxes which include police, fire, water, schools, etc. These damn parks suck communities dry, then leave those who pay real property taxes with the bills. There has been legislation to try to raise the fee to $50 a year and it didn't pass.