He shits on weak laners like Talon or Kass. He'll just dance in the middle of lane while you're pushed to tower.
However, buying boots can solve that. You've no idea how much trouble I have with Kat. Not that I fear her burst, but the fact that sh starts with boots. When you have few things to focus on, a good melee can evade everything Xerath throws at you in lane.
His Q winds up and can be side stepped, his W can be straight up out ran, E is solved by either going ham and in his face or by hiding behind minions. If he tries to E you then he's more than likely on the far right or left. Tell your jungler this for easy money.
His ult... Well, it's hard to aim and relies entirely on Xerath predicting where you'll be when you're 100% evasion mode.
Also, since the mana regen nerfs, his spells cost a truck load. Mana's a weakness for him, and since he builds max CDR, Qs and Ws for days.
He cannot build a tear because that delays his power spike for too long, so he needs to rely on mana regen which works only if you've a high mana pool. He needs that passive because it puts him in his crappy AA range to gain just shy of 200 mana at max rank. That's less gain than a Q W combo would burn.
Yes he has insane DPS in teamfights and burst if you get some poor sap in the jungle. But that comes at the cost of having to manage his mana. Does he try to poke now with just Q or does he wait until the teamfight starts to start button mashing and combing everyone in range?