Well, things like shootas and lasguns wouldn't worry you as much. With each model being at least T5, most infantry weapons would have trouble just getting wounds, wounding on a 5 or 6. Meanwhile, high S, low shot weapons (plasma, melta) run the risk of just being outright denied on a +4 save no matter what. Literally the only weapons that can ignore their +4 is an
AP 4 or better weapon that ignores cover. And aside from a colossus, these are mostly flamer units. That have to get close. To orks with antitank machine guns and power klaws that have effectively an 18"+ charge range with fleet...
DE on the other hand have so much poison its not even funny, and would be able to put out the volume of shots they need and would still wound normally. They're one of the few instances where I would worry about being outgunned. Not to mention that ork bikers wouldn't have the numbers or the hitting power to wipe out their serious units in close combat. So it would become a game of cat and mouse, trying to outmaneuver one of the most maneuverable armies in the game. Would be fun, but definitely an uphill battle.
Also, I have a list thread in the list forum where I'm tossing list ideas, if anyone feels like seeing some terribad lists and needs a good laugh

. Trying to nail down what I will work towards for my initial 1,000pts list, and how much of it will actually be bikers, and how much will be support units.
http://www.dakkadakka.com/dakkaforum/posts/list/476125.page#4765706