The way unified_inventory does things makes it a joy to work with for developers, while more modern inventories like i3 are flashy, but do not support as much and are harder to work with. Registerable buttons and fairly easy configuration make this mod very flexible, and add-ons like Unified Inventory Plus make crafting a charm.
As for criticism, Unified Inventory desperately needs a better sorting system, although VanessaE's newest one is pretty good - but it needs a bit larger support. A more modern look might also help a lot. Also, UI does not come with a good skins mod out of the box, you need to install something like u_skins to get a nice selection of skins.