I've played AC odyssey and Valhalla previously and mirage is much smaller in map size and game play. I finished the game in about 30% of the time it took me to play odyssey and 50% of the time to play Valhalla. You get much less game with Mirage, which I found very disappointing.
They also removed a lot of the weapons such as archery and melee. All you have to choose from is a sword and dagger. Also far fewer skills to be learned in the skill tree, and they really aren't that cool.