Why would you ever take Ivan Dashkov when you can take Bia, Earl, Harry or Hatice? The big guy's a slow behemoth, good at dishing out damage in the early game, but vulnerable to speech attacks and, if you choose him as a captain, you get saddled with a low Resolve. The fact that he never gets a double attack means that long term he does not do as much damage as many of the supposedly 'weaker' characters, and although he eventually does get Primal Roar, his lower Speech Power means it will not be as effective as Dolores' Roar. His speech abilities are inaccurate and if you take him as a captain your only reward is to win Aggressive for +1 Encounter. Not bad, but is it worth a sub-standard leader? My take on Ivan:
-Early game his Melee attacks can crush many enemies with one hit, but even typically aggressive fighters will often switch to Devious against him and suddenly he's nearly out of the fight.
-His Primal Roar does lower enemy Attack by terrifying them, so if hostiles reliably use aggressive attacks against you, it can slow down groups. Only rarely will it take out targets unless you really buff his Speech Power.
-His main ability, Pinning Strike, is a Boss stopper. It deals good damage and fixes an enemy in place for one round, leaving them attacking Ivan. Good if your team has ranged attacks- you can pin a boss then shoot safely with the other team members.
-He can get a weak area effect ability with Collateral Damage, if you don't mind him wading into a bunch of enemies.

Not great. If you can't run him in close, he's useless. If you do, he's a big blundering target. At least Bia is fast and has Grit! Ugh. Well, for what it's worth, some strategy ideas.

-Like Bia, go for Swords and Followers that grant more benefits from an Aggressive win. Stock up on good Attack and Speech Defence raising items as soon as possible.
-Even though his Primal Roar is weak, it does Terrify multiple targets, making them more vulnerable to Harry's pistol or Anna/Earl's Terrify. Of course, you're often too slow to get into a good position to use it...
-Your benefit for winning aggressive (+1 Encounter) means bonus Campaign and Gold, and you can stack more Gold with Followers and the Adventure research abilities. But the maps that give you the most Gold early game (Caribbean, Egypt, Mali) are filled with enemies who are effective against Ivan... and you're low on Resolve.
-Harry is useful for clearing away smaller enemies that might take him out. Harry is always good, really. Primal Roar and then a Piercing Shot from Harry is one of the few good Ivan combos.
-Charles is great at drawing attacks to himself, away from Ivan. Ivan can run in, start whaling away, and then Charles' group insult draws all the resulting Speech Attacks to him instead of vulnerable Ivan.
-Icon of Nicomedia, (Transylvania) Roche's Pirate Hat (Emerged Island) or the Celtic Horned Helmet (Celtic Highlands) all stack well with his captain ability. Good luck getting the first one.