With respect to Born probabilities, TI is on the level of MWI, it has no derivation for them. Similarly, its ontology is rhetorical rather than rigorous.

A central issue for any zigzag-in-time or retrocausal theory of QM would be vacuum polarization, which was the stumbling block for the most serious effort, by Feynman and Wheeler. But Feynman-Wheeler theory is also where the path integral was born, so TI advocates could say, we just need to go back and finish it properly.