Thanks Tattie, ok a few tests / questions.
Q) If you try merging any of these meshes in a new scene does it crash?
Q) If you merge 1 or 2 of them does it crash?
Q) Is there a specific mesh which causes it to break?
It would help in finding the crash if we could have those specific meshes especially after the above tests.
Since the merge is a new system, there is bound to be problems that we missed lol

Or it could be as simple as a .mesh file file setup or more lol
