I think it comes down to: Can Armstrong block a bolt?
If he sees it coming, yes.
But I thought that Ezio also had a gun?
Yes, but it's something like the first gun ever and is pretty much equivalent to a small round crossbow bolt.
See, Armstrong's war experience wasn't your "boom goes the dynamite" kill everything missions. He fought in a guerrilla war, where people were doing sneak attacks all the time. He'd be much better at spotting an opponent than a guard.
And there's another thing the supporters of sneaky-sneak Ezio are forgetting: the setting. In most situations Ezio has tons of people, and buildings to use to his advantage when sneaking about. The reason the guards are so easy to sneak up on is that they have no idea who Ezio is, and they see hundreds of people walk by in the span of an hour. One hooded guy in a cape isn't that exciting to them. Armstrong, on the other hand, would flip out as soon as he saw another human being.
Ezio's style wasn't the stealth most of us are used to. He was not seen because nobody ever expected him. He hid in plain sight, by being the most unassuming guy possible in that get-up of his. All he has to help with that here are trees and bushes, and while he's fast enough to duck around corners he's not the most quiet guy.
And in direct melee, Armstrong wins period. He's fast enough to dodge any blow Ezio can use, and strong enough to probably dislocate or even break bones with his punches. The spiked gloves help too. If Ezio is using metal armor, Armstrong could probably transmute it to a very cumbersome form (that is, HIS FACE) and eliminate what protection the assassin has from his blows.