Lots of things. Among them:
In many jurisdictions, they're treated as personal property, not real estate. Sometimes, the titling is done through the state's department of motor vehicles.
Manufactured homes generally depreciate, again similar to vehicles. Not as fast, but they do typically depreciate.
Often (in parks) you might be buying the home, but you're renting the land it's on. That's typically called ground rent or a land lease. And that sometimes can be a larger monthly expense than purchasing the home itself. (Example: I sold a manufactured home a few years ago, providing owner financing. The owner monthly pays $210 on the mortgage and about $850 for the land lease.)
Newer manufactured homes (I forget the year of manufacture) had to meet fairly strict construction codes. Older ones had looser requirements.
If you're buying in a park, the management firm may have fairly strict requirements about who can purchase. That may involve both a credit and a background check.
It can be difficult obtaining bank or conventional financing, unless the home is new. Many sales are accomplished with owner financing. Either that, or they're cash sales.
Depending on where you are geographically, many probably won't be listed on the MLS. Where I am, for example, I'd guess that maybe 20% are on the MLS. The other 80% are sold by owner. So while a Realtor may be able to help you, you should also drive around--through the parks themselves--to look for homes for sale.
Manufactured homes can be a very cost-effective option. Compare the total price of ownership (mortgage plus land lease) to what comparable apartments in the area are going for.
Any other questions . . . just post them.
Hope that helps.