So within the community there is a large disagreement on Kislev's Power. I'd place them near the bottom tier, being one one of the worst races in the game. It's somewhat hard to come up with a race I consider lower tier than them. Vampire Pirates being one.
A big reason for disagreement comes down to the Legendary difficulty bonus being -8 control(devotion.) Since Kislev needs to get high control to activate their bonuses, the -8 control from Legendary means they can almost never activate it. Even with commandments, technology, heroes, etc. it's very difficult for Kislev to get to 25 control to even use their bonuses. This means compared to their lower difficulty friends, Legendary Kislev players can go entire campaigns without MAJOR bonuses being used.
To put this in perspective, on Legendary it's fairly common for it to be turn 40+ before a single devotion bonus is activated. You have -8 control from legendary, and -4 control from collecting taxes. That means it takes +12 control JUST to break even, and +13 control to get just +1 control a turn. You need 25 to finally spend it. Ending turn at +1 control means you will definitely be done with a campaign before you get up to 25 from where you were early in the campaign.
Therefore, I recommend Kislev gets -4 control from legendary difficulty, instead of -8 control. With lower difficulties being reduced as well. Potentially, Kislev could lose the entire penalty to control from difficulty in order to make their faction bonuses not get reduced (much like how Bretonnia works.)
Personally, I think Kislev would still be low-mid tier with this change, but I think it'd at least be a start to try to equalize the different difficulty levels with how Kislev performs.
To keep this in perspective from people who don't understand how strong Kislev's bonuses are: spending your Devotion (control) can be 50 growth and +3 recruitment capacity for 5 turns, or 8 melee atk and magical attacks for the entire army, or 50% character XP + flammable attacks + flaming attacks (enemies get -20% fire resist, and you get fire damage.) These 3 are powerful to get almost for free on lower difficulties.