You simply cant get backstabs on everything my char is a 30x backstab, and you can't do it on most tough creatures, Giants, mammoths, dragons etc. they will detect you even in the black of night directly behind them with 60 sneak and full + sneak gear.
Go with a thief/mage. I sneak attack with a single dagger (x30 from the DB gear) and if I can't kill it in one (or two quick) swipes, I cast Invisibility, position myself behind 'em, then when I'm fully hidden and they've relaxed, I sneak attack again and usually waste them.
Oh, and yeah the Ninja idea is cool, that's why I went Thief/Mage (Illusion and Alteration). After I finish with the DB I'm going to train him up on dual-one-handed weapons and he'll be ready for anything.