The biggest problems I have with my Kakuzu use is that EVERYTHING he does leaves him open. As far as his U2 being safe from midrange... I kind of disagree. I've used it midrange against a Sage Naruto and he blocked, U2d and it hit me before Kakuzu finished. His tilt is also VERY dangerous and VERY hard to get to the third motion. If someone subs his first move, you become their ***** for about 2-3 seconds (actually, if they sub just about ANYTHING he does). If someone wants to be a real ass, it's pretty much possible to sub anything Kakuzu does and then U2.
I agree however that his air combos are quite good, that's mostly what I use (as using his ground combos is about the equivalent of BEGGING to be abused). They also go into knockback. However I'm starting to doubt the ease of landing the U2. I played Bobroham the other day and in one match he subbed 3 knockback-U2s then abused me. Granted you can time the sub on them I'm sure, but 3? That's a lot.
Overall though I think he's a very underestimated character and not many people use him (or can). He does have some good moves, it just takes a lot of timing and risk management to use them effectively. Plus I consider his U2 to be the most absolute badass U2 in the entire game.