Kairos is fine too against most models in the game. Sure thing Legacy is better, but Kairos is Lv 50 and he do his job just fine as for me.
I can agree on that, he is better than Ayl to be sure but it still hurts he doesn't do well in solo duels for his 45 points.
As for goethia. Yes that is all true ut let us not forget that Kairos is a melee attack and enemies can manouver themselves via 2 free inches during combat when they see that Kairos is coming. Second Kairos has fewer chances of being buffed via Sophia unlike Goethia due t him geting close and personal. Give his whirlwind +1/+1 and he will start rock'n roll'n but for now he has to rely on bless at least to be scary. Because really with all these aura heals, from gemini, lilian, easy to put shield buffs, if the aoe doesn't connect well it becomes useless