I don't want to get your hopes up, because Dany vs. Westeros probably will be pretty disappointing, but keep in mind her army is basically the worst army ever when it comes to siege. Dothraki are horse nomads. Reavers are pirates. They have no supply train (since their holdings are across the ocean) and indications are they're going to be a foraging army during winter. The dragons are great, but there's only three of them; they can't be very many places at once.
Realistically? Her army should fail, spectacularly. They should hit their first major castle siege, strip the nearby land bare of forage, then starve to death from lack of supplies while having no real meaningful way of cracking a hard inland target with their horse-archers and pirate ships.
That probably won't happen, I'm fully expecting a steamroll due to Deus Ex Draco, but in a "realistic" situation she actually has a pretty shit army for the war she wants to fight, not counting the possibility of being gifted a kingdom outright to use as a foothold.