It's not a theory that makes quantitative predictions, it's more a blueprint for a future theory, and the critics would say that the blueprint is hopelessly flawed - that no such theory is mathematically possible.

The larger theoretical context of Lisi's work is the attempt to describe 4D gravity as a gauge theory, the viability of which is the central dispute between string and loop theories of quantum gravity. Lisi's theory is a "GraviGUT" theory which then adds to this problematic foundation, even wilder hopes about getting fermions from "BRST ghosts", and about finding loopholes in theorems which say that even then, you couldn't get the necessary three generations of them, out of a single E8 gauge field.

Incidentally, there are various ways to get three generations of particles "from E8" in string theory, so perhaps those should be regarded as the real "E8 theories".