Not a geek, but from my own experience with mostly freeware aircrafts...
What do you mean by "do not work"? Do not display at all, do partially not display, display but various functions problems, etc...
I have been flying FS5, FS9, FSX, and now P3Dv4. Under the later opus, I had to abandon any hope regarding FS9 aircrafts, as well as FS9 ported to FSX. Generally, FSX native aircrafts do work OK, except some "active" elements (mean .gau, .dll files for instance) which need to be rewritten or recoded in 64bits mode. There are also some problems with textures (prop disks, alpha channel, etc...).
I guess this might be related to the software version and modelling type used to create the aircrafts, but can not confirm so far.
For instance, amongst the aircrafts that I had to place for good in the museum area are the MAAM-SIM R4D, Oliver Fischer Bücker Jungmann and Ju52, George Diemer amphibians and flying boats, Milton Shupe Lockheed Harpoon, SOH A-26, and various french aircrafts of the 1930-1940 era...
Blue skies