I have played with Kroak a lot and I have played against a fair few Kroaks while playing around with some FEC & Orruk Warclans.
I can see the frustration, I played a game last night where in one turn he did 39 mortal wounds in one turn, from very far away, with +3 to cast on everything. That output from one model sitting pretty much wherever he wants on the table, on a 3+/4++ with 10 ablative wounds near him is very oppressive. There are counter plays - you can snipe the support pieces (though their often screened and have a 6++). You can try play around his 22" Celestial Delivarance, though Arcane Vassal makes it hard. On top of his MW output, he supplies command points, board wide unbinds with +2 to unbind commonly and he him self is very hard to kill.
Yes his normal outfit is a lot of points, (Balewind + Banner + a vassal) but the banner bearer brings fantastic utility him self, any skink wizard hero is fantastic in it's own right. For 320 points Kroaks warscroll is pretty wild (4 casts, board wide dispels, +1 built in, fantastic warscroll spells, acess to stellar tempest) etc etc.
It can be very demoralizing to see your army eroded away without much interaction, and that's where peoples frustration comes from. While there some counter plays, some armies simply don't have them. Kroak in FoS with skink screens some armies might as well not play the game and even those other armies that are in the S/A tier with Seraphon are really only having a fighting chance.
The reason why Seraphon are top tier is really a longer answer but TL;DR would be - undercosted units, huge mortal wound output, very strong shooting, hyper mobile, teleports, ability to move outside of their own turn (In FOS). They have everything you need to compete, dictate the pace of the game and play the scenario every game while offloading huge damage.