Upgraded Partisan can take a few hits before going down.
This is the only OP part, and it's an accident. The health upgrade gave Partisans 50% more health back when their health was 40. Relic raised their health to 80 (standard for all infantry) in the march deployment but forgot to remove the upgrade from the anti-infantry partisans. So as a side-effect, anti-infantry partisans can be upgraded to 160 health, the highest in the game. But like I said it's a mistake so expect it to be removed in the next patch.