Dawn of War Codex

Why haven't i seen anyone plays karskin, what prevents players from using them? are they purely a situational unit?

The main reason is that a leman russ is so powerful and cost-effective that you should save your resources for at least one of them and not bother with kasrkins.

The other reason is that, for their cost, Kasrkin are not very effective (They feel like a really bad attempt at an all-purpose unit for the IG). If you get plasma kasrkin you end up with a unit that does slightly more ranged dps than a GM squad, but which costs substantially more (both in up-front cost and in reinforcement cost). If you want piercing DPS, you get storm troopers (which have more DPS and are cheaper). The only time I get kasrkin is when I want their grenade launcher - it does laughable damage, but it has passive knockback on each attack, which makes it handy for controlling big nasty melee units like a nob squad. It is particularly handy near the end of the game, when the VP counts are low and your enemy sends beefy, hard-to-kill units like nobs to cap VPs - you won't kill them with the grenade launcher but you will prevent them from capping a VP, potentially buying your team precious time.


EDIT:

Here is a thread where I show a comparison of the DPS/cost of GM and Kasrkin: viewtopic.php?f=3&t=2700

All I know is that in the SM matchup a Leman with 2 IG repair is god mode. SM AV dps is just too low unless buffed las cannon Dev that doesn't get tied up or T3 las Pred is present.