Playing alone vs. playing with a group probably depends on the playlist. If you're really worried about k/d, playing MLG or Arena alone probably isn't a good idea. In BTB, invasion, squad, TS, etc you shouldn't have much problem playing alone if you're at least decent. Playing as a random and beating the piss out of a full party of higher-than-eclipse players in TS or Invasion is a favorite pastime of mine. You might not be able to win the game with a crap team, but if you're just there for fun or for kills, you should have no problem achieving personal success.
I play alone or very rarely with one other friend, and my k/d is what I'd call "acceptable." It all depends on what you're doing.