When a team is properly running the "spread" offense, it doesn't take much to make the defense look inept. One particular concept is "levels" wherein a group of receivers, maybe a boundary and a slot, are joined by a motion RB or TE. At the snap, they all race downfield and break in at different, you guessed it, levels!
This sounds straightforward, but keep in mind that, when breaking across a field horizontally, the QB can throw to either arm of the receiver (that is, hit him directly with the pass or lead him further upfield). In order to blanket 3 receivers, you'd need 6 DBs! Clearly, the defense can't commit those resources! Using rotations and check offs, a defense can put a lid on levels if it collectively recognizes it quickly enough, but it's not uncommon to have one guy be slow reacting (usually a guy who had other primary responsibilities like covering the QB option or inside give running play).
Texas has a lot of work to do on its defense. No doubt. However, LSU really knew their stuff going in to that game. I'm not overly concerned about the Longhorn defense, so long as they continue to improve with game experience.
Army is a different animal. They do one thing really, really well. If you can't handle that one thing, then they bleed clock, yards, and points out of you. When you're used to having 7 possessions to fix your mistake, you're probably only going to get 4. In that way, they really take advantage of an opponent's offensive miscues. Rice doesn't have the horses to play both run and pass, so they were grateful to Army for picking one.
Texas shouldn't have any problems with Rice. They lack the athletes to cope with a multiple attack. Just try to stay healthy.