With Xros Wars you have to turn your brain off and forget the power levels of the Digimon originally as they can be as strong as Megas or as weak as Rookies depending on what role the writers thought for them. It's not always for the best, fans of Digimon like Marsmon and Huanglongmon will gladly tell you how disappointing their debuts were, but it was mostly done to give fun fights and they can be enjoyable.
That said, one flaw I will note with this system is that even within itself Xros Wars can be inconsistent. Sometimes a strong form can go up against one of the big bads, then the next they're getting smacked around by random MotW. JetMervamon vs. Harpymon is probably one of the most infamous example of that as she was going up against Death Generals in the last season, and then in the next she takes a knee from a MotW and she's out.
That said, one flaw I will note with this system is that even within itself Xros Wars can be inconsistent. Sometimes a strong form can go up against one of the big bads, then the next they're getting smacked around by random MotW. JetMervamon vs. Harpymon is probably one of the most infamous example of that as she was going up against Death Generals in the last season, and then in the next she takes a knee from a MotW and she's out.