Let me put this in a different perpective:
Option A: U-turn
You use U-turn and you switch out no matter what.
Option B: Ghost-typed U-turn
You use this move and you switch out unless the poke has a Curselax (well hi free turn to curse), Blissey or Togekiss (who counters like most of the CAPs around). If the opponent has any of those you lose your best scouting move. Just like that. Add the small possibility of being hammered by CBLax Pursuit and you will have a very very fun (read: bad) situation.
This proves that, if you want to see the switch-in, Bug-type U-turn is the best option more often than not
Also, this poke was based on the entire purpose of forcing switches. Even if you pack your poke with 4 varieties of U-turn, it won't be worth shit if you can't force the opponent to switch. And how do you do that? You do that with powerful moves and, on the standard tier, powerful moves = STAB. Having reliable STAB moves goes before U-turn in terms of importance, and how do you expect to forces switches with a steel typed attack that is resisted by every bulky poke (read: Bulky Waters and Bulky Steels)around and deals super effective damage to only 2 types? Scizor can only do that because it usually packs either SD or CB to go with its 130 Base attack and a benefitial nature, while your every day CAP7 will have neither more often than not.
If you want to force switches you will need more STABs to play with. More STABs and better STABs (again, the only reason Scizor actually forces switches with U-turn is because it has 130 base attack, adamant nature and CB or SD and LO, while our poke will be lucky if it has LO and max atk). 60-70 STAB is NOT good enough. Just look at the numbers of dusknoirs that force switches with the threat of shadow punch (same BP as rift hop). This is why we need a move with at least 80 BP to serve as STAB (same as Dark Pulse, Crunch, Iron Head and Shadow Ball to give you some examples) and force switches.
To sum things up:
- This poke have no issues with detecting the switch-in, since U-turn is already a terrific option. Don't try to sell this ghost U-turn move with that argument, 'cause it is false, as U-turn, even without STAB, is the superior option, not being nullified by any type and all.
- The problem this poke do have involves reliable STAB.
- A Reliable STAB usually has at least 80 BP. Shadow Strike has that AND a great secondary effect that will net you a handful of extra 2HKOs. Therefore we need more Shadow Strike than Ghost U-turn.
Oh, and for this amusing "the opponent can switch in skarmory and make your efforts useless" argument I will give you two fun facts:
1) If you are so afraid of Skarmory, you can use Bug-type U-turn and go out. The only difference between it and the ghost typed is the difference of getting out and leaving Skarmory untouched after Leftovers and getting out only if the opponent doesn't use a normal type and leave Skarmory with 96% hp. Since getting out is the priority, Bug type is the superior option.
2) After the defense drop you can actually 2HKO Skarmory with LO Thunderpunch (standard Skarmory receives 49.10-58.08%). This means that if skarmory is stupid enough to stay in, it will only be able to get one layer of spikes down and if it moves out, congratulations! you have forced out one of the best physical walls in the metagame!