IG are actually probably the worst thing DE have to face. Against massed AV 12, dark lances aren't exactly amazing. Multilasers glance and pen on the same values against their AV10 vehicles as the DE's lances do against the IG's AV12 tanks, but have three times the numbers of shots and inflict greater damage due to DE's vehicles generally being open topped. Other anti-infantry weapons can also be put to good use against DE vehicles, such as Heavy Bolters.
If you work out the math, chimeras versus even a Ravager aren't too far from an even match in a shooting war. Given that IG can get two Chimeras for one Ravager, it makes it very uneven.
Also, Multilasers and autocannons, two weapons very popular in IG armies, not only are exceedingly effective against DE vehicles but also inflict ID on DE characters, ignore FNP on most of their units and wound just about everything on a 2+, usually ignoring saves. Then of course there are dirt cheap Hydras which not only have very effective anti-light infantry/anti-skimmer guns but also ignore one of the DE's most powerful defensive actions, which is moving flat out with skimmers.
Additionally, DE splinter weapons wounding on a 4+ isn't very impressive against T3 infantry, they're basically AP5 lasguns at that point.
DE are very, very good at destroying small numbers of very powerful things, they make mockeries of terminator armies and stuff like Land Raiders and Monstrous Creatures. They get butchered when they have to face large quantities of mediocre things.