It's worse than Garchomp in the sense that it's faster (whilst still being in a unique speed tier), and can get even luckier with regards to hax (60% flinch > 20% evasion). However's it's SLIGHTLY better than Garchomp because of the lowered defences and a SR weak. That said, it's still bulkier than all fast special sweepers (except scarfed ones like Jirachi and Togekiss, but then it's more powerful than they are), so it's hardly FRAGILE.
I don't see immediately what the Regi's are going to do to the SubSeed set.. maybe I'm just being naive, but if they switch into either Sub or Seed they're going to be walled for an eternity. The possibility of Air Slashing from behind a sub is just the icing on the cake really. The Subseeder can also scout Tran's speed level if it subs on the switch in, letting you see whether it will outspeed you or not. Crobat is also shaky against this set as well, because it won't be able to switch into a sub more than once before Air Slash catches up to it...
On the note of the SubSeeder, wouldn't Wide Lens be a possibility? I know the lefties recovery is great and all, but with everything but Sub having less than 100% accuracy, Wide Lens could really help... 93.5% Seed Flare, 99% Leech Seed, 100% Air Slash... could be a good "surefire" way of getting it to work.
I was also thinking of trying to get Substitute onto one of the offensive sets, but then the item choice is a lot tougher. Yache Berry (which needs its own set BTW - it's a viable item and it hasn't been listed) doesn't work with Sub well, Life Orb kills Skymin far too quickly, Expert Belt lacks coverage... leftovers seems pointless on a sweeping set, and Lum Berry is pointless when Sub blocks status. The best I can come up with is Salac/Petaya Berry... but something like this could be viable:
Name: Sub Sweeper
Move 1: Hidden Power [Ground]
Move 2: Seed Flare
Move 3: Air Slash
Move 4: Substitute
Item: Petaya Berry/Salac Berry
Ability: Serene Grace
Nature: Timid
EVs: 252 SpA / 252 Spe / 6 Sp.DEF
Note that this would be best with a 30 IV in HP, so that you could sub 3 times to hit the berry (or twice and a SR switch-in). Petaya would be the better choice because it's already more than fast enough for most threats. Subs on the switch, and works from there. If a Regi comes in, hit it until the sub breaks, switch, and come back later to terrorize again. If Tran comes in, you have a free turn to HP Ground it, since it will probably expect a SubSeed varient anyhow. If it's not faster (although it most likely will be if it's the first thing to come in), you still have a sub intact after the OHKO. Crobat won't be able to take this thing on more than once if you have sub up as it comes in, so switching out after the first hit isn't exactly a bother.
If you manage to come in enough times to sub down to the berry, more power to you - by this time you should've already done a LOT of damage anyhow, but the extra speed/Sp.ATK should just seal the deal. If tying with other Skymin isn't your priority, and you like your 31 HP IV, you could run 12 HP/ 252 Sp.ATK/ 244 Speed just as easily, but the 30 IV allows at least a tie with other Skymin... something that could be important thanks to the unique speed tier.
EDIT: Sorry, this was in construction for a while, so the top part was slightly outdated -_-. Some of the rest of this post has already been said too, but that's what I get for taking a while to type this out :(