Appreciate the confirmation of Cauldron! I very well may take up your offer of tips, once i dig in properly.
For the most part? Yes. I'm currently having issues with mobs not spawning correctly in mushyisland biomes, as well as not spawning as often as they should be anywhere else -- but I haven't pinpointed why. I could just be doing something wrong. In any case, it's something that can be tollerated if you're not trying super fancy shiz.
I heard about the mushroom island spawning, though, those islands if I recall have a really derpy code around them spawning in Oceans, that are different to 'normal' land spawning (if I am remembering, the world gens as all ocean, then X % must be land, it picks a spot and slaps a continent in). Mushroom islands it last I saw, checks after continents, and generates it not as a 'blob' continent, but a specific island mass. Not 110% sure that would be the issue (and not much I can suggest to fix it), but yes.
Not spawning as much as you'd like, is PROBABLY the configs. I've skimmed them, and christ - they amount of cross over and redundancy of stuff (like weighs per biome, despawn times/distance, etc) make this a technical nightmare to test. But I'm going to assume the issue is likely the distance of mob despawn (as it's keeping mobs 120+ blocks away loaded, and it doesn't despawn for X time, which if I recall 600 ticks=60 seconds). More so you'd want to try and make despawn be quick, and instant despawn be short ranged, like 50 blocks (about what a player will see immediately). this forces more spawns, closer to the player generally, or should in theory.
But yes! two questions I had remain to be answered, if anyone can help?
#1 How to define configs on a per-world basis?
#2 if there is ways to alter mobs variables (damage, health, etc) and create 'alternate spawns' (zombie 1, zombie 2, etc with unique health, damage, all that) and if not, a mod that plays nicely with JAS that can do that.
Appreciate the info! Hope I can help others once I sink into this thing.