Yeah I want this thing ou too.
I find that it can do nothing at all to tran, registeel, ect. People need to get off the mentallity that it needs to have a wall that can kill it for it to be ou. It gets seed flare (8pp) and air slash. PP of seed flare is easily stalled by switching around and once its gone blissey is a sure wall.
Scarftran can outspeed, not be ohkoed, and ohko before skymin can touch it. The only set with the power to take blissey is the specs one which is countered by smart switching between heatran and blissey.
Once paralyzed by blissey, It is easily revenged. Also sharders such as wevile and mamo can come in to revenge. Also if locked on air slash, tyranitar can com in, threaten pursuit, and stone edge it.
All of these reasons make it viable for at least testing in Ou.
1) "It gets seed flare (8pp) and air slash. PP of seed flare is easily stalled by switching around and once its gone blissey is a sure wall."
Do you know how crippled your team would be by the end of the 8 turns? And that is if the Shaymin user doesn't predict to Sub-Seed on you, or pull a Growth... By the end of 8 turns, especially with Growth, you're basically dead. Even if Blissey is a sure wall, some physical sweeper such as Dugtrio can just take her down, and you'll basically have a team full of 50% HP's facing a team of Growth or SubSeed Skymin with no Seed Flare PP, still plenty to Flinch the heck out of what's ever left in that Shaymin, and kill the rest with the other 5 while you're just feeble.
2) "Scarftran can outspeed, not be ohkoed, and ohko before skymin can touch it."
It must be +Speed nature, and even though, simply just switch into Infernape (will not die from Flamethrower), get a free Nasty Plot; Salamence and Dragonite as well, and pull a free Dragon Dance; and of course, Tyranitar, which also pulls a free Dragon Dance or Rock Polish, then start having fun with those as well... Nobody said you have to keep Skymin in on Scarftran. Also that is assuming you don't switch into HP [Ground].
3) "Once paralyzed by blissey, It is easily revenged."
No! Again, just by having a Dugtrio on the team, Blissey is nothing. Immunity to Paralysis, and CBTrio is guarenteed 2HKO on a standard Blissey with Earthquake, or OHKO with Crit. If you predict something like Ice Beam, then switch into Hitmonlee. Again, immunity to Paralysis and great Special Defense, OHKO with Close Combat...
4) "Also sharders such as wevile and mamo can come in to revenge. Also if locked on air slash, tyranitar can com in, threaten pursuit, and stone edge it."
Mamoswine/Donphan/Weavile all get OHKO, I believe by Life Orb Timid Shaymin's Seed Flare, so you have to sacrifice one just to switch something in.
5) One point you didn't mention:
If you switch BLISSEY into Seed Flare, and has its SpD lowered two stages, it'll just die to a succession of Air Slash flinches... There is no safe counter. Period.