I understand that you can tackle Batari or Ull in either order, but given that the Izila have better health and higher damage output, it's clear that they were meant to be tackled last.

This to me was a mistake. They could've at least made them equal. Maybe Udam are harder to kill and Izila are easier to get killed by? That would make sense given the Udam are based on the hardy Neanderthal and the Izila are based on the relatively advanced Mesopotamians.

The reason I'd prefer this relates mostly to story. The Udam are dying out, and once you learn this there is a real finality to defeating them. They'll never recover from Ull's defeat. Meanwhile the Izila are just some crazy cult that shares the exact same DNA as the Wenja. When Ull dies, it's like your closing a chapter on the Stone Age. When Batari die's, it means nothing.

Halfway through my second playthrough, I'm actually grinding skill points to kill the Izila first, and then I'll notch up the difficulty when I go after the Udam to create the illusion that they're the true endgame.