Default Settings: When I build a long chain of hoppers and let them drain a single chest into a double chest the double chest will eventually fill up if you refill the single chest from the double chest.

disableEntity = true
disableTileEntity = true

I hoped that at least this would be spared but no. Using a dispenser with a redstone clock and throwing items into a nether portal while repeatedly walking in and out of it will eventually give you a single dupe. This is very unreliable and slow, giving you one item every five minutes (I didn't measure the time) but it shouldn't happen.

In my opinion world parallelization should be fool proof. It's the bare minimum to make this mod useful. The number of cross world interactions in vanilla is quite small. Small enough to warrant manual patching of the vanilla code. I am thinking patching nether portals, end portals and some commands that let you teleport across worlds should be enough.

If world parallelization works properly this mod could reduce the need for bungeecord. The next step after that would be to use chunklocking for vanilla to fix the hopper problem. Multithreading everything sounds nice in theory but in practice even vanilla doesn't work properly.