From what I recall, they have a new model format that uses JSON files to define the model. Sounds great. Should make designing custom models more accessible. Sure it is, for vanilla. Then, throw in mods that like to do things beyond cubes, suddenly, this new model format is a clunky old dinosaur. One of the modders calculated their mod would need a couple of gig of json files in the new format, and that mod isn't one of the major custom block users.
So in an ideal world, forge needs to add in a true custom block renderer (as in, restore the old version) so modders can totally bypass the json mess.
Then, add on top of all that the usual mixed reviews on performance being worse, even with all the "improvements" to the rendering system.