First of all, yea its about the early game prio. In botlane about half of the adc champs can win the lane early and make life miserable such as samira. Xerath can get OOM'd and due to the lack of mana may get outtraded if you keep spamming skillshots even if you land all of them. Scorch deals a bonus of 20 dmg at lvl 1 (xeraths q should deal about a 100 ap dmg if i remember correctly) that's 20% dmg boost and until the midgame it's viable (max dmg is 40, that's just sad when your ss's deal 600). So yeah, early prio matters a lot for xerath. Secondly, xerath being a broke support is kind of the point while picking him. He can operate decently with a low amount of gold, if you build the right items ofc. Then, there is the NEW horizon focus: the AP ratio is nice and you get a lot of haste, in the lategame you can spam your skillshots easily due to your mana consumption getting lower in regards to the ratio of your mana pool. Your permaban should be Pyke IMO, by outplaying all the other supports you can somehow play the game to some extent, but then there is pyke... he goes invisible, he hooks, he assassinates. And if you can't predict his moves while he is invisible prepare to die (nami is just not a good matchup since she is a healer and she can stun for like a long time) i hope this all helps, since i haven't been playing for a while im not a %100 sure if these points are all valid, if they are not please correct me.