Ben was very commited to make a FSX/P3D compatibility list for many addons out there.
While most of the sceneries from FSX will work in P3D, with non-native products there always might be some issues (e.g. Gary from UK2000 told me that PAPI lights at EGLL will or might not work – still to be checked).
Nevertheless, sceneries free of DLLs, can be usually ported. Sceneries like ORBX, FSDT, need to be adapted. They contain dll.xml entries, which need to be P3D native and compatible.
Furthermore, be aware that the product publisher might have issued an EULA preventing a product from a licensed usage in P3D.
Also, a column is provided (coming soon!) which will regularly be updated with the info if the developer has given their consent to porting his product manually into P3D. The developer can say no, but that also doesn’t mean it doesn’t work. Just that you should not do it and probably would be unwise to post screenshots of doing it.
If you know how, most addons can be ported to P3D, but many will need manual transfers of files. This is where FSX2P3D Estonia Migration Tool comes in handy. It will disguise P3D as FSX and allow virtually any FSX addon to be installed into P3D. The question is only if the addon works afterwards.
The compatibility list by Ben can be found here: