Tzeentch is the shooting/ magic god. Their units should be like that. It's always been the case where all the gods are specialized, and taking a pure army of a god forfeits the advantages of the others.
Nurgle is hard to kill, but is slow and can't do a lot of damage
Slaabesh is fast, but fragile
Khorne is great in melee, but has no magic support and is also not super tough
Tzeentch is resilient (in different ways then nurgle, shoots and casts magic.
I don't think tzeentch should have combat monsters, as that takes away from Khorne.
I think it'd be good to wait for the codex to come out, as strategems, rules and points changes can make a massive difference. For example, if splitting doesn't cost points, it'd make pink horrors very valuable.