Ap trynd has been seeing increasing amount of play recently, but he has a lot of things that are so bad to play against:
1- Having a great burst with a great DPS;
2- Having so much unrestricted mobility, being able to dash everywhere for no cost and no cooldown at all isn't fair at all.
3- So much sustain that can't be affected by GW, as he will just walk away and heal back up. Furthermore the heals are insane!
4- Being so hard to itemize against, as he does physical damage with his 1.0 AP E (which is a bit confusing to people who don't know his kit), and dealing magic on hit damage too! (so mixed damage, with some true damage as he will go conqueror....)
5- Not being punished by over extending, playing the map wrong or anything at all, even hard cc is barely a counter to him (one or 2 can't cc him for long enough as he will afk split push your nexus)
6- Takes tower so damn fast.
7- Not being punished for using his ult, as he will always have it back up (with 45% CDR it is always off cooldown...)
To people who don't know what is AP trynd, he will rush nashors (and will get the ap parts first) into lichbane, rageblade, deathcap and other situational items. His E cooldown is 3.8s, and if he gets some lucky crits here and there it is shorter.
Also right now AP build is stronger than the AD build (according to Fogged)
I hope he will get some balance changes to shift this power and give a better window for counter play.