Converting non-infinite maps from 1.7 creates this boarder I can only call "garbage" that is reminiscent of the old Farlands bordering from legacy versions, but hanging over a void.

Sometimes, when it successfully converts to infinite, it will be below sea level with a massive wall or water surrounding the map. 

One 12.2 map converted into chunky blocks like a puzzle.

I've tried to first open these worlds in the version they were created in or various in-between versions. While 1.14  & 1.13 convert the world edge at a normal generating speed, earlier versions rendered the garbage area with severe lag. I've tried this on several old maps by a few different creators.