(legendary edition, pc)
At level 42, first game.
So far I've had Uthgerd, Jenassa and Serana as followers and they've been useful. They help in fights, cover my back, distract enemies when I'm fumbling around. They tend to be over-aggressive (attacking before I'm ready) but I can compensate for that since I know they'll do it.
Now I try Durak. He mostly just watches. I provoke a wolf. let it gnaw on me . There's Durak a little bit away, watching. I move towards Durak. HE BACKS OFF!! I keep moving, eventually the wolf is next to him and he crushes it with one blow. This sort of stuff happens over and over.
In the same situation the other 3 followers would have either charged the wolf with a melee weapon or blasted it with a staff effect (Jenassa is quite fond of dual wielding staffs). Serana would have frost-bolted it from extreme range, animated a nearby corpse AND summoned a fire atronach.
I can't get Durak to use his crossbow - at all.
He started with the default crossbow. I got a Enhanced Dwarven crossbow, smithed it to "fine" and gave it to him. He's never fired a shot with it. He *has* stood around while an archer pumped arrows into him and yelled at the archer. But he didn't take his crossbow and shoot back. After many shots he charged the archer and killed him. After *many* shots. (he has many bolts for ammo)
(besides the unoffical bugfix mods the only mod I'm running is one that makes unread books glow)
Then I tried Ingjard as a companion to compare with Durak.
She used the crossbow (both normal and Enhanced Dwarven), quickly intervened when something attacked me ... in other words, behaved just like I expected a follower to act based everybody *but* Durak. I'd use Ingjard but she's not a good match for my sneak-based build. Durak isn't ideal but he seems to be closest match among the non-level capped followers.