My family has been a retailer of manufactured homes (sometimes modular) for nearly seventy years and we own and operate five trailer parks. I’m by no means an expert, but here’s my opinion.
A trailer on a pad will depreciate, regardless of how new and modern it is. If you can get the land for a hell of a deal and the land is already prepped to accommodate a mobile home, then it might be worth it for a cheap mortgage for a while... but you most likely won’t make any money down the road.
A trailer on a basement or stronger foundation typically holds more value, but that is dependent on varying circumstances. It’s a huge gamble.
A well built modular home (different from a manufactured home) is no different than a stick built. My parents purchased a brand new modular home twenty years ago from a manufacturer for 99k. They put it on a basement, added a two stall garage, large porch, and had an addition built. Their house is currently appraised for more than 300k (which is expensive for our region). People are surprised to learn that the house was delivered in several pieces.