trust me, it's just b/c opfor didn't pack enough AT weapons.
land raiders are very cool, but off on their lonesome even with servitor back up it's vulnerable to meltabomb assault marines or warpspider haywire bombs. warpspiders are potentially worse since i think when they're upgraded for haywire their filament guns get more effective vs. vehicles. (on the upside they die faster compared to assault marines) but in either case 2-4 squads of them will probably really mess up your land raiders.
2 land raiders represents 8 units of vehicle support. for the same support levels you can take 4 dreads. i'm pretty sure the dreads would beat the raiders in close combat. the combined lascannon might destroy one of the dreads, but then the other three would probably quickly finish em off. OTOH the req cost is probably lower for the double raiders, also dreads can't transport anything (but does anyone use that ability much?)
on to another disappointment. fire prisms. they're pretty cool, but that splash is murder on your own troops esp. if you go heavy melee