Well, like it was mentioned I just assumed that having a really strong (comparatively to others on its level) digimon would result in a significant increase in backlash, not to mention the more difficult path to get there. Uhm, I guess I could use T.K. and Patamon as an example of this. It took them significantly longer to achieve the Angemon digivolution and it only lasted moments before Angemon reverted to an egg. So, in my case there would be this really strong Omnimon X, but chances are it would take Rhodey a lot longer to even get that digivolution for Mistymon, added on to the fact he wouldn't be able to maintain it as long, and it would result in the massive setback of being reduced to baby 1 level.
This is just my opinion, but I would be very surprised if a WarGreymon could handle something like an Apocalymon, AncientMermaimon, or Omnimon (well, Omnimon is a combination of WarGreymon and MetalGarurumon, so yeah). The Mega/Ultimate level is tricky, mostly due to the additions of DNA digivolve and a handful of other circumstantial digivolutions. Though, the bonus for WarGreymon, I would imagine, would be that it can maintain its Mega/Ultimate status longer than some of the aforementioned digimon, and the backlash probably wouldn't be as bad.
Off-Topic: Anybody else having server issues at the moment?