Here's a list of all of the oddball sceneries that I've been able to get to work in P3D v4 with their default installers or the migration tool. I didn't do anything with folders or changing files. These are all "out of the box" installs:
Latinvfr: MIA, SAN, RDU, TJSJ, SNA- Everything works but the autogen buildings surrounding the airports have black walls and normal roofs. RDU looks great, all buildings seem fine, but there is an option to turn off surrounding autogen in the installer, and I used it. I can deal with the funny buildings until they come out with their v4 versions.
Tropicalsim: MCI, DCA- Loads and seems to work fine.
SXAD: BNA, no problem if using the LP folder instead of the HP one- I could never figure out the difference, but the HP in v4 gave me a CTD. LP fine.
Taxi2gate: VHHH, KMCO, KSEA work fine. SEA is by far my favorite addon scenery- maybe because I can see a coffee shop inside the terminal from the ramp. VHHH is just fine, but I really wish they would update the scenery as the construction of the terminal is finished and the airport is undergoing another expansion. But, it's still much better than the default.
WCI: Narita- Fine, no problems.
Performance: All of the airports seem to do better in v4 than v3 performance-wise. I have my sliders mostly to the far-right, except autogen stuff which is at high, and most of the water reflections are off. I did not do a lot of day/night testing, so there may be issues with lighting that I don't know about yet. I'm just happy that it looks like I won't need to sacrifice my route network as I ease into the new platform.
Please don't ask me for tips, tricks, technical information, how-to's or anything else. You can learn through trial and error or scour these forums for all that kind of info. But, if anyone has been hesitant or curious about these sceneries, they all seem to be just fine so far.
Happy flying,
John