Its not true, yes Khagan is slow but he is hard to. His mana buff is helpfull.
You said he is dead or nit ready when fight end.
What with Isarnia/Azlar? They both very good SLOW heroes, they both more squihy than Khagan. Yes, I know they dealing much more dmg (+fire/debuff). They have 100% offensive skills.
Khagan have something like 40% offensive and 60% deffensive skill.
Mix him with healer like Rigard/Melendor and after he buff himself (and nearby), he can survive massive dmg.
WhencI have only him as maxed 5* he was my tank. With him and slow/average 4* I was at 2250-2400 cups.
I think he should be buffed but still, he is 7-7,5/10 5* hero. At 3^70 he is bad, thats true (his dmg is a joke and he dieing too easily) but when he is maxed he can show his usefull.