Large FVF allows batching of many custom shaders, but should not join items which have shaders that utilize BUILTINs which would change for each item, because these will not be sent individually, and all joined items would wrongly use the values from the first joined item.
Batching is mostly separated into a common template which can be used with multiple backends (GLES2 and GLES3 here). Only necessary specifics are in the backend files.
Batching is extended to cover more primitives.