Again, they may not have shadow tag, but they will take out a chunk of your team.
Taking a chunk of damage out of something is not the same as a KO, especially one you can't switch out of.
As said previously, Garchomp will severely cripple whatever switches in that does not resist his Outrage.
This statement is far too vague to be taken in literal context, so let's propose a scenario. Before Garchomp uses Outrage, the following conditions have to be met: a) it's boosted by SD, making it relatively safe to use, or b) you've properly scouted your opponent's team and know that it's the safest/only option you have to adequately threaten what's in front of you, as well as what might switch-in.
This is not the same as getting a kill with Shandera. For one, its strongest move doesn't lock it in, and on top of that, you don't have to worry about what switches in unless the target is carrying a (rather obvious) Shed Shell. If Nattorei or Forretress have Shed Shell, you'll either need to predict by using another move or set up a sub to make sure you can deal with what's coming in with WoW, Shadow Ball or something. Keep in in mind that the incoming Poke cannot switch, even if the matchup becomes unfavorable.
The only reason why I can see we may have to ban it is that the user can "select" what to take out. Then again, prediction comes into play, especially when the opponent knows that you're running a shandera. A smart opponent will bait in a Shandera, while switching out a counter.
Eh...see, the inherent problem with arguing prediction is that it's inconsistent; you can play any ol' goofball on ladder, execute said strategy, and say it works, but it's a different ballgame with regards to two experienced and equally skilled players.
For example, I'm using Scizor, I can abuse his slow U-turn to push prediction in my favor then bring in Shandera to kill whatever you send out. That or I can just wait until a point in the game where you can't afford to predict and switch it in on the safest opportunity. See how quickly this gets us nowhere? Prediction doesn't work as a valid argument unless it's within reason.
Again, your argument about set up can be applied to Garchomp too! Switch in on an electric move, that does absolutely nothing, proceed to Swords Dance on the switch, and will "probably get more than one kill." Same logic here.
I really don't think you understand how Shadow Tag works. If Garchomp switches in on an Electric move, the Pokemon using it can switch out. Once it does, Garchomp is facing down its check/counter. That means it's not getting a kill at all.
Against Shandera,
you do not have that option. Shandera gets a free kill because the Pokemon it's threatening cannot switch. If Chomp gets a free kill because your opponent didn't have a decent way to stop it, that's dandy, but it's not the same thing as keeping anything you come in on from switching, then subsequently killing it or setting up and killing other Pokemon afterward.
And food for thought: if X has broken properties and so does Y, then there's a striking possibility that both are similarly overpowered. Such is not the case here, however, as Garchomp and Shandera have two entirely different roles and capabilities. Any comparison drawn between the two of them would be loose, at best.
Overall, Shandera is a mediocre pokemon with an incredible ability. Too bad most of its best counters are already high OU. The popularity of Tyranitar and rain teams are going to ensure that Shandera doesn't get too out of hand.
I find that statement pretty ironic because Shandera is the Pokemon I tend to use to kill off other weather starters. Once your Politoed takes a Specs Energy Ball or Tyranitar takes a Specs HP Fighting, you're pretty boned.
