Mobile home owner here. I'm pretty sure that most of their bad reputation as an investment is because most of them do not come with the land, but if they do come with the land, then as an asset they will behave more like other real estate. The main thing that is valuable about a home is that it confers the legal right and practical ability to live in that location, but mobile homes placed on rented land categorically do not.

To address the other points (warning: c/dull_mens_club style content ahead):

To give an example of a repair task related to custom appliances: The water heater broke, and replacing it was complicated by the differences between normal and mobile home water heaters; the latter are smaller, and the cold water intake pipe is on the bottom rather than the top. I had to downgrade to a slightly smaller, mobile home approved tank because the previous one was a regular water heater too large to be up to code as it is placed in a small contained closet and there are clearance requirements. The floor underneath it also needed some repairs, which were fairly simple. All of the plumbing, electrical wiring, and joists underneath the floor can be accessed from the crawlspace by cutting through the wrap material and insulation with a utility knife, and then stapled/taped up when done, which is a little awkward given the lack of space, but everything is accessible without that much difficulty. The cost of the whole replacement was mostly the tank itself, which was more expensive than a comparable non-mobile-home water heater, but only by $100 or so.

Overall, the thing is a really simple single story structure, and compared to what I saw my parents go through trying to maintain an old farmhouse it seems like easy mode.