Hammerfell had the Alik'r Desert to drag the Elves into, so they can fight on their own terms. The Redguards had a MASSIVE advantage over any Thalmor
And the Redguards are notorious for being the best warriors in all of Tamriel
I know, and Skyrim would be just as hard for the Dominion to invade. There are few viable ways into the province for an army.
To attempt a naval invasion, they would have to travel through the treacherous Sea of Ghosts, and the only waterways into Skyrim are the Karth River and White River, which would be death traps.
Eastmarch is protected by the Velothi Mountains, which the Dominion would have to traverse through Morrowind to get to. Riften is protected by the Jeralls. The Reach is mountainous enough as it is, and I doubt the Thalmor would be able to go through Hammerfell and the Dragontail mountains unscathed.
That leaves Falkreath, a huge pine forest. The safest route that an Aldmeri Army would have would be walking right into an ambush.