Togekiss and Jirachi are slower than Skymin and they both lack a move - Seed Flare - with an 80% chance of lowering the opponent's SpD 2 stages. They lack a sub-seed abuse tactic as well.
How exactly? Skymin has more SpA and Spe, way better type coverage and similar bulk. It's only real disadvantage is its Stealth Rock weakness.
Okay:
Togekiss can use twave to outspeed and paraflinch. It can use its bulkiness and nasty plot to OHKO faster pokes, and air slash to stall out walls. It gets aura sphere and flamethrower too.
Jirachi has great defensive typing. It gets Iron head, which is more powerful and accurate than air slash. It hits more types SE too. It has a huge movepool to abuse with serene grace, and it's STABs are much more useful than skymins. It can run a mixed set too.
Skymin Vs. Jalorda: The speed difference is neglegible. The only notable pokes that sit between them are starmie and darkrai. They can both subseed effectively, but because Jalorda only needs to run leaf storm and hp, it doesn't get moveslot syndrome like skymin does. Crobat, aerodactyl and jolteon all outspeed both and hit super effectively too.
Shay can't run seed flare, air slash, earth power, hp fire/ice, sub and seed all at once. You're going to get walled by
something.
Meanwhile, Jalorda only needs leaf storm, hp and sub. unlike shaymin, it can sweep reliably, and even subseed without hindering itself. Glare, twister, giga drain, torment, gastro acid, taunt, screens, swagger... Pick whatever you want for the last slot. Its 75/95/95 defenses are arguably bulkier, and it's lower hp has better synergy with leech seed.
Now, the STAB moves. Let's compare them:
Seed flare. 120 power, 85 acc, 5 pp, 80% chance to lower one pokes' spdef.
Leaf storm. 140 power, 90 acc, 5 pp, 100% chance to double Spa.
Can shay spare the turn spent using growth? Will you have sun up?
When jalorda attacks, it gets a free nasty plot thrown in. It's special attack is raised, meaning it can sweep an entire team with a boosted stat.
Kill something with seed flare drops and you're back to square one. No boost here.
Skymins' higher spa stat is irrelevant. If it has the time to use growth, it uses up a moveslot and gets 508 special attack. Jalorda just has to attack (pick off a weak opponent for example) and its special attack hits 498. attack again and you get 747. At +6, jalorda has 996 special attack. That's 3 turns just using its powerful STAB move. It's also high enough to render its tiny attacking movepool moot. Any neutral coverage move will destroy almost anything.
I'd like to see almighty uber shaymin pull that off. The only reason it was ever considered uber is the result of the hype its STAB moves caused. It's a legendary, it looks great on paper and it's clearly much better than it's land form, so it went to ubers.
It's really not that broken. There are much more threatening pokes terrorizing OU than shay.