TLDR: proper use of sylas's kit takes too long to get off to build full ap without being instantly killed.
im sure by now everyone has seen sylas's kit and thought "well the ability to steal ults is fucking bonkers" and well, your right the potential is there to be game breaking as hell.
but the issue here is that the sort of builds that could cause these sort of issue dosent work very well with his kit, or rather how quickly his kit goes out.
his kit while having decent ap scailings it has fairly low base damage and high mana costs for things that are ment to be spammed with max cdr built. (his damage falls off the face of the earth if he cant do a rotation so max cdr is a must on any build).
his skills are also fairly telegraphed while his engage takes a LONG time to go from sylas to his target.
realistically sylas going a full ap build will be niche lane build or his only jungle build as getting his full rotation off takes too long to get off. within that timeframe he could be bursted down before even killing one person if he even manages to get his e off without being killed before getting there.