It all comes down to the question of keeping the vent dry. Vertical pipes are better because there is nowhere for the moisture to linger. It flows away, leaving the vent clear of traps. However, many wall vents have a 90-degree bend to allow the vent pipe to extend above the roofline.
If debris falls into the pipe, the bend is the weak point where blockages collect. If that happens, water becomes trapped, and the vent no longer fits the purpose. It’s also more difficult to clear because the angle inhibits a plumber’s rod.