Although I can harvest synthetic crystalline, since it started spreading, I can't clear even a single square of it. I had a part of my map set up as a sort of crystalline farm (pic attached). No number of tundra rovers can make a dent in it, and the problem has coincided with a significant amount of lag. If I delete a road or wall adjacent to crystalline, it is almost immediately filled in with spreading crystal.
I'd guess that this can be replicated by creating a large area of synthetic crystalline surrounded by roads and\or walls.