Jungle Edge M Biomes are not very well known, I may have seen more JEM (Jungle Edge M) Biomes than most Minecraft Players. Here is some of my Observations on JEM Generation.

1. JEM Biomes usually generate along side Jungle M's, but can Generate on Rare Occasions along side other Modified Biomes, mainly Swamp Hills.
   
   
2. JEM Biomes are Bigger when bordering Swamps or Swamp Hills.
   
   
3. JEM's have a 0.000451% chance of Generating.
   
   
4. They have become even rarer in 1.13 for the change in terrain and Biome Generation, like in the case of the Seed Cold Taiga.
   
   
5. They can sometimes generate over 500 blocks in length.
   
   
6. They aren't always mountainous.
   
   

That is all of my Observations on JEM Biomes, and I will have a link to a Photo Gallery of Images that show these observations.
Enjoy!

The Photo Gallery: Gallery