Regarding the later years.
A) What are shields?
B) Halo has Spartans and personal shields? First of all the weaponary used by the Empire would decimate those shields like the covenant weapons do. And Spartans are by no means invincible.
C) Actually they could handle the rebels. Just not a Force user (though not a full jedi) with a full technical readout of the Death Star and was a great pilot. And the only reason Luke survived was due to the idiocy of that one tie fighter pilot. Darth would have just killed Luke there and then and sorted Han out.
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A) Those little glowing things around your Spartan and Elites. And, if you are to go off Halo Wars, some vehicles. B) It would take longer to kill them, and being that this includes the Covenant, they would be facing a group with much better armor, too. I don't recall Stormtrooer armor actually serving a purpose in terms of defense. C) So the Empire lost due to its own idiot soldiers? They already can barely hit a Rebel, let alone Luke.