Most likely because of the Post from RSR (PMDG)
https://www.avsim.com/forums/topic/512651-14jun17-how-not-to-mess-up-a-perfectly-good-prepar3d-v4-installation/
I suspected your concern was related to that post, and not something that LM said...
Yesterday I tested a Scenery compiled with the SDK from FSX but caused an instant crash after loading the scenery.
It's not possible to say if the crash happened *because* the scenery was compiled with the FSX SDK or that it might have been prevented if it was compiled with the V4 SDK. It might have been just be a single file, or a single texture, or a missing texture, so that's just an anecdotal case. Not possible to say without checking that scenery.
I don't put your skills into question, in fact, I trust you more than other developer, but what PMDG states also makes perfect sense to me.
It's just wrong to generalize. Each scenery is a separate case, and I don't even think many sceneries out there are compiled with the FSX SDK, most of them use the FS8 SDK for the ground polygons, and the FSX SDK for the rest.
But that's not enough to label a scenery as "bad". It depends which instruction codes were used in each SDK. Some of them are still perfectly supported, some aren't, some cause a performance loss, and most of the unsupported or not well supported scenery codes are the ones from the FS8 SDK. In fact, we were amongst the few scenery developers that got rid of FS8 code for all our products released after 2010.
Not even the default scenery that comes with P3D V4 is all compiled with the V4 SDK. In fact, most of it it's not so no, if the usage of previous SDKs to compile sceneries was enough reason to make the sim unstable, LM itself would have recompiled the entire default scenery with the latest tools, which is not the case.