There are many reasons why U-turn is undesirable, and Shrug has listed the major ones:
1. Circumvents the risks involved with switching - you will always get the upperhand or at least not take major losses from switching out
2. It punishes opponents from switching, even though that's the most desirable option otherwise
3. It exacerbates entry hazard damage by forcing many switches (Deo Spike-stacking VoltTurn Offense is as brainless as it gets)
4. Forces many 50/50 scenarios that broke multiple Pokemon
5. It is widely distributed, allowing a team to heavily abuse VoltTurn with high success
Reasons 4 & 5 are the main reasons why we're pushing for a test on U-turn rather than Volt Switch / Baton Pass
Basically U-turn takes skill out from the match by giving you many safe switch-in opportunities and favorable match-ups with very little to no cost. All the while it adds chip damage, unlike Baton Pass, and it cannot be blocked by immunity, unlike Volt Switch.
1: Yes, it does circumvent the risks with switching in some scenarios. But it doesn't help with them at all a lot of times. Lets take the simple example of Scarf Jirachi vs. Dragonite with MS broken. The opponent has a Heatran and a ~50% Scizor waiting in the wings. There are two options for the Jirachi user. Either switch out using U-turn, or Ice Punch. It's a 50-50 gamble, and if you guess wrong Dragonite will set up and do major damage, or against Ice Punch Heatran and Scizor will come in for free. Baton Pass is the same as U-turn here.
Lets compare that to Scarf Keldeo (for arguments sake we'll assume rain isn't up). With Keldeo, you can either Surf or Icy Wind. Once again, if you guess wrong, you are in a bad spot. I fail to see what makes U-turn so great here. Obviously U-turn shows it's real power when it can be used with literally no downside against Lati@s, Alakazam, or Celebi, but if you think it is broken then don't use physically frail bug-weak Pokemon.
2: It does punish opponents for switching. So does spamming Specs Hydro Pump or Band Close Combat or using Hydregion. So do entry hazards. Half of a game of Pokemon is about trying to punish opponents for switching.
3: It does exacerbate entry hazard damage, but this is a 50-50. The U-turn user is taking entry hazard damage as well, possibly even more since all the abusers bar Rotom-W are Spikes weak and all bar Jirachi are SR neutral. If you have Spikes + SR on your side of the field and the opponent's side is clear, you're at a major disadvantage no matter what they are doing.
4: Okay, like I've said before, Genesect was not broken because of U-turn, most voters said that the RP set was what made it broken. Tornadus-T was broken because it had like 3 counters who all lacked recovery, plus wasn't punished for simply dry switching into a counter for their Jirachi or Rotom. Landorus is possibly the only Pokemon that was banned because of U-turn -- and as Panamaxis said, that's because Bug/Fight/Ground/Ice coverage is unwallable, not because of U-turns switching effect.
5: It is not widely distributed. The abusers in OU are Scizor, Jirachi, Tornadus-I, and Landorus-T. Thunderus-T and Rotom-W are the two Volt Switchers. Hydregion has much better uses of its moveslots than U-turn -- there's no point switching out of a counter when you literally have no counters. Gliscor has a weak U-turn and generally is strapped for moveslots anyway. Infernape sucks and should be UU; Victini and Darmanitan are much better (although these two do abuse U-turn very well, they are quite hazards weak and sun-reliant). Celebi prefers Baton Pass to U-turn. Xatu uses U-turn well to use the slow switch out of threats, but might as well be using Baton Pass for all the damage it's doing, and probably would be using Baton Pass > U-turn for Ttar if it had access to it.
Scizor is great because of U-turn, but Jirachi's U-turn is weak, and really only used on the certainly not broken Scarf version. Landorus-T handily uses U-turn to pivot. Tornadus-I hates using it because it is losing 25% of its health each time, and should really just be spamming Specs Hurricane when it gets a rare switch-in chance since very little walls it.
I agree with Panamaxis that this thread is silly. This is basically a "Scizor is broken thread" since I don't think anyone would argue that (U-turn) Jirachi or Landorus-T are even close to as powerful as Keldeo or Kyu-B or Loom or Terrakion or Latios or the other threats in BW2 that are more worthy of a discussion than this move.
U-turn was much better in Gen 4 when you actually needed to scout and such. It was much better in BW1 when those damn VoltTurn teams were everywhere. Nowadays, it's not too great
EDIT: Also, for those saying that the argument isn't about U-turn's abusers, you are wrong. Yeah, if Kyu-B, Terrakion, and Tyranitar got U-turn I might think about banning it, but they don't so we have to look at what actually can use it.