Stakh has a really nice game but just lacks in some areas. Doesn't seem to have enough upper body strenght to generate more power to his shots.
I was thinking the exact same thing. Part of it is he hits flat and he doesn't really hit into and through the ball like most players, he doesn't throw himself into shots. He's more of a guy who tries to do damage by timing the ball perfectly, but unfortunately he doesn't have the weight of shot required to do enough damage from the baseline and that strategy is hard to execute, which explains his inconsistency.