The Nexus had a very defined storyline vs. Cena though. The Sheild's storyline likely involves a dart board of babyfaces back stage. One week it's Ryback. The next week it's Team Hell No, then Sheamus, then Orton, then Miz, now Rock... no rhyme or reason. That's not really a lasting storyline. It's just any random face to get a heel reaction.
The WWE doesnt have to make them look weak in a program with someone. They dont have to be squashed. Clearly they are on a different playing level than 3MB.