As mentioned, the best part about Boyars are there Melee abilities and high moral. I normally dont build them, the Russians normally have enough princes as they are 105 florins maintence each unit

That Melee ability has saved my butt more then once, as they rarely break and there are few units in the game that can go toe to toe with a boyar unit with even a avarage general.

I always struggle with horse archers as they are a pain to run down, Boyars arent fast so thats a problem to. For me I keep 1-2 steppe calvary units in my army that are direct support for the boyar units. There cheap and fast and are expendable. I send them in engage horse archer units and let the boyars widdle them down with bows at the same time. Yeah I loose the steppe calvary unit to friendly fire sometimes but at 188 replacement cost its not a bad way to protect your boyars.