It's pretty darn hard to call the move itself broken, considering that it is a 70 base power move with a mediocre offensive typing. Not only this, but Scizor is basically the only pokemon who hits notably hard with it against pokes neutral to it, considering most other U-Turn users don't have STAB on it. I can see the reason why one may think that the move is broken, especially since a few of the latest bans have used it to frickin perfection (Scarf Genesect, LO Tornadus-T, 4 Attacks Landorus-I), and the most used pokemon in OU, which is Scizor, most notably a threatening move on the Choice Band and Choice Scarf sets.
The large problem with Choice Scarf Genesect was that you could never just fix your EVs to limit it's "Downloading" capabilities. If it got an Attack boost against your Celebi / Lati@s / anything else weak to Bug, it basically either forces you out of the field, or knocks out your Psychic type. If you do choose to switch out into a resist, your opponent couldn't care less, since he gains a completely free switch into a check or counter of choice.
The large problem with LO Tornadus was the fact that 1) Hurricane hurts, 2) almost nothing could take a Hurricane and a U-Turn comfortably without being a fully specially defensive pokemon who resists Hurricane and 3) being a fast U-Turn user with Regenerator, Torn-T could just U-Turn out for free. Hurricane + U-Turn had almost no risk (back at full HP) and had the massive reward of taking off a good >50% health from the majority of OU.
Landorus is just way too threatening. The way it used U-Turn was to take care of it's biggest problems which where Latias and Celebi. This means that it can use a "free momentum" move whilst gaining critical coverage. The list of pokemon that could comfortably take any 1 (if faster) or 2 (if slower) attack(s) from Landorus are as follows: Gengar, Latias, Latios, Bronzong, Chansey, Blissey. 3 of these pokemon are mediocre in this metagame at best, whilst Lati@s are hit hard by U-Turn on the switch.
But the thing about these threats are; U-Turn really just made life easier for them. A good player can easily outplay the opponent without using U-Turn on them, and U-Turn just made the whole procedure an easy no-brainer. This argument can be used both for and against the banning of U-Turn. On one side, you could say that U-Turn, the move itself isn't broken, just the pokemon that utilize it well where, and on the other, you could say that U-Turn is unhealthy for the quality of the metagame, as it honestly requires no prediction skills, and is basically just a "click me for free momentum" button. I definitely would not classify U-Turn as an "Over-Powered" move, but the whole fact that it just makes the game so easy to play just encourages me to consider it unhealthy for the metagame.