He's one of those caster models like Kairos or Nagash that some bad RNG basically makes your 430 pt investment useless. If you roll double ones on his first cast he can't do anything the rest of the hero phase, that point sink would have been better on say a skink chief on steg that has tons of melee power and can even shoot. Granted that doesn't always happen, but I'm finding a lot more success with a Slann with the Arcane Might command trait, prevents the chance of a miscast pretty reliably.
In terms of competitive lists, the only real nasty thing I've ran with kroak that works well is a Fangs of Sotek double skink cloud list, two units of thirty skinks with buffs from priests and starpriests supported by kroak and a slann with the astrolith buffing casting gets some serious magic done. that alpha strike list will run right some shoot things to hell and then you can summon on the back line and start getting skink speed bumps or salamanders and teleporting them behind enemy lines. That list is fragile tho since it's so dependent on rng.
The thunderlizard lists I've been running forgo infantry for lots of monsters, engine, chief, two battleline stegs, and a bastiladon do serious work with way less support heroes. Really only need a starseer, slann, and skink priest for the command ability. I played a lot of lists with kroak to make him work but I feel like he's not a must take for us like some armies have with their centerpiece models.
It sucks but he's just too random with casting and his death mechanic and he's honestly so much fun to use. I've ran him with a slann who takes celestial equilibrium, combine with the astrolith and the sages staff I got a +5 to cast on my rolls, only a snake eyes will fail a cast. This reliably gets ten or higher on comet's call too. Only Lumineth with their auto unbinds or Tzeentch with their highest roll is both shenanigans will be able to contest a bonus that high. Definitely try Kroak out but with the monster bonuses this season I feel like thunderlizard is our better faction, and Kroak doesn't do much for them imo.