-Crossbowmen, Gendarmes, Khazar Royal Cavalry: none of these guys is useless, but all are made redundant by a small upgrade.
-Ballista: can't hit anything smaller than a wall, and for sieges there are better options.
-Trebuchet/Mangonel: to weak, a catapult is cheaper and is much more accurate.
-Viking Thralls: see Demon of Light
-Abessynian Guard: they should be powerful Gazi, instead they are just expensive Gazi.