I play slann and kroak together in dt all the time so I've had a lot of experience with both. Like at least 75 games this year I'd say. We play all the time and have monthly rts. I feel Generic slann is bad for these reasons:
1. Slann is a 3 cast wizard that doesn't really have good spells to cast. The best spell in the lore is stellar tempest, which is situational. The comet's call spell is also situational and best when finishing off units on their last wound. Celestial equilibrium is the next best spell in our lore, but it feels really bad using a spell to make another caster better because you have nothing to do with your magic phase as a generic slann. Kroaknado build is really the only reason I'd use that spell. The other spells in the lore are pretty weak and I don't find myself ever wanting to use them. So really you have to rely on endless spells, which you pay points for. That 260 wizard casting bound Geminids is really 340. A generic slann for 340 is pretty bad in my opinion.
Kroak on the other hand, has 4 spell slots and a single purpose. To summon balewind vortex and spam mortals 3 times onto people. He is always using his spell slots and dealing mortals. This is a big deal. You pay premium for magic in AoS, and wizards that cannot make the most of their magic are really weak.
2. they have board wide dispels, but the game is moving away from magic to shooting. Paying a premium to be masters of magic when people are either throwing bodies or bullets at you and foregoing the magic phase entirely, means you are wasting points.
For the generic slann this is half his identity. Half the reason you take him. He is there to dispel and cast spells. He is supposed to be a big wizard. Kroak on the other hand is there to do one thing, mortal wounds, and the ability he has to dispel is a bonus. I'd take kroak even if he couldnt dispel because I want his offense. I can't say I'd take a slann who couldn't dispel. It would feel too weak.
3. he doesn't really support the troops you have or allow you to project power on the board. He just sits there as a beacon, making command points if you roll well, sacrificing worthless spell slots you can't use for summoning points and allowing models to be summoned near him. He's more of a terrain piece you pay points for than a hero model you take into battle.
This contrasts with kroak who wants to be in the middle of the board, is constantly dishing out mortal wounds, killing enemy heroes to remove their buffs and weakening units to allow your troops to finish them off. Kroak also carries a giant target on his back, drawing fire away from your own troops to him, which I can say the slann has never done in my games.
4. generic slann has one good use that I've found and it's as a support hero in DT. You take generic slann, make him the general, bring along arcane knowledge so you can cast equilibrium to buff kroak while keeping a healing spell in the back pocket. Use the slann as general to generate summoning and then sacrifice spell slots for more. In this build he will make his points back through summoning and helping kroak do kroak's job even more. Its the only way I've found to make the slann feel useful.
kroak on the other hand is always useful because he's so focused. Do I want to take mortal wounds? Yes I do. Ok I'm taking kroak. He doesn't do much else besides spam mortal wounds and that's ok, I always feel like I'm getting the most out of him. I don't need a big elaborate strategy and setup. He's simple and will slot into anything.
so yes you can argue that kroak makes the slann obsolete by being better, but really it's the slann is weak. He's hard to fit into lists because he doesn't being much to them. He's pricey and actually not the best at the role he is supposed to be played. Rather than nerfing kroak, the slann needs to be buffed. He needs to be bringing more to the game. That will encourage people to take him more. As is it is too difficult to make him feel worthwhile and that is why kroak is seeing so much ply, because kroak is simple and straightforward and always getting work done.
Appreciate the response, but not sure I totally agree. Feels like you gloss over a lot of the strengths with some pretty surface level complaints, at least in my opinion.
1) A 3-cast/3-unbind wizard for ~85 points per cast is good. Period. Almost none exist in the game, and your assertion that comet's call or stellar tempest (2 of it's 3 spell slots) are "situational" is arguable at best, flat out wrong at worst. Comets call is an UNLIMITED RANGE mortal wound spell that doesnt require los. Even if it hit one target that would be amazing but it has the potential to hit 6! It's easily in the tier of "best spells in the game." Stellar tempest also single handledly gives seraphon options against horde. at 24'' minimum you aren't going to outrange it and can easily do 20+ mortals over the course of a game.
Paying for endless spells as a downside also is kind of unfair, considering bound geminids is criminally undercoasted and again, probably one of the best spells in the game.
Between those 3 spells seraphon can pressure 1) armies that rely on out of range, low wound heroes. 2) armies that rely on tough, large blocks (mortek, dok) and 3) everyone else because bound geminids is literally amazing always.
2) Magic is still at a premium. Seraphon, Lumineth, Cities, Skaven, DoK, and tzeentch all really, really REALLY want to get spells off. Unlimited unbinds is literally the best part of a slanns kit, and this opinion sounds heavily skewed by local meta. Almost all the best armies in the game besides KO and fyreslayers want to get spells off.
3) You said it yourself, he makes command points. A lot of command points. That's arguably a better "support" resource than spells. Also, we've already talked about stellar tempest, comets call, and unlimited unbinds. What exactly are you looking for this wizard to do?
4) All you've done is argue that Kroak is better. And he is, he's a lot better. It's why in a world where kroak is 320 you never see a slann at 260. But that doenst mean a slann is bad, and hand waving away comets call, stellar tempest, command point generation, and rangeless unbinds to make your point, kinda makes my point. All of those things are pretty awesome IMO.
Your opinion also sounds a little warped in that you are always playing them together. A slann is, by definition, a support piece in your list. You're criticizing him for being a "terrain piece you summon around" when that's exactly how you treat him. I've played dozens of games with only a slann, and he gets along just fine on his own. I'd suggest giving it a spin and leveraging the points you save from not taking the kroaknado to getting more toys.
edit: i will say, i'm fully aware i may be a bit biased and a full on seraphon apologist. I think the book is pretty great top to bottom and am reluctant to really call anything in it "bad" outside of a few extreme examples.