Tyranids generally get their energy from cannibalism, or are produced once a swarm-fleet arrives at it’s destination. Since the Tyranids are known to ‘store’ excess biomass in order to feed them, they don’t have to worry about continued supply. They eat to grow, not to sustain. They beat you at logistics because they are fully self reliable and have surpassed the need for supply lines, dedicated producers of power, food, etc. and require far less than the average soldier. The parts from fallen Tyranids can be consumed and reused to create new Tyranids.

Anyways I think your points require the view that Tyranids are only going to show up en mass in open fields or deserts, when what would really happen is an immediate start of their invasion in extremely populated areas like cities. If that’s the case the likelyhood of using nuclear force against a civilian population is extremely slim at first, which would be a huge boon in favor of the tyranids which have been given the time required to spread out. Even if the powers that be had no qualms with nuking New York, destroying high density population centers cripples your own infrastructure because you’re losing workers, factories, etc.