>>39463364Silvally has three issues as I see it: No item, no ability, and a lackluster movepool. The first issue only applies if you're not running the normal-type variant.
Silvally's typing, an attribute most other mons get intrinsically, instead takes up both its item and ability slot. So you've got to commit to running a mon that has functionally no ability and cannot carry an item. Its like trying to battle using rules from Gen 1 against someone playing by Gen 7 mechanics.
Its movepool has some interesting features, like Parting Shot and Tailwind, but for the most part suffers. It surprisingly lacks Earthquake despite being quite large. Lacks Extreme Speed which Arceus lords over it.
For a mon that you would think in theory would be very versatile because it can change its type, it is the exact opposite of that in reality.