For one, their is such a big power gap in Digimon evaluations compared to Pokemon. By champion(adult) digimon are demi-kaijus, easily capable for warping landscapes easily. One example that comes to mind is Seadramon. Of course, aside from normal Pokemon, the legendary and others of close power would probably hold out well. But with the bound between Digimon and their tamers, one would be quite a force to be reckoned with against Digimon.

One of the reasons why a digimon would be stronger than a pokemon is that in the digital world they fight for their LIFE. Unlike in the pokemon world where they fight for a competition. I am just comparing the "mon"( without legendary pokemon or mega digimon) by their will to fight a digimon will kill for the sake of there partners (see Digimon Adventure 02). And we cannot forget the fact that digimon have WEAPONS! A fight between a gun and pokemon, the gun will win. One last reason I will say that digimon are smarter. there can be arguments about this but digimon can speak with human on a intellectual level. Pokemon are closer to animal (legendary excluded) they behave like loyal pets.