His downside is he literally cannot survive large damage champs. His HP cap is 2471 and the scaling of that HP puts him on average behind every champ in the game but adc, because they all build SOME HP. And Thursday they are reducing his HP cap to 2301.
He generally has like 1100 HP when other champs have 1400-1600. It's also VERY prone to kits that have % Damage built in, because they have minimum damage. For example,
's cleaver scales up to 280 Minimum Damage.
Any penetration game utterly destroys Pyke.
His Ult is heal-outable, disruptible, stasisable, revivable, clonable, side step/dashable, not near as easy as you let on. You can literally destroy his ult mid cast.
His damage output is also fairly low. Even though he can scale up to about 560 AD, the typical AD is about 330 mid game, and he generally only gets about 20k damage dealt in games where allies have 30k or better. His cooldowns cap his ability to mete out damage.