Air is supplied within the SubCom.
That's fine and dandy, but the atmosphere around the SubCom could still get into the Mech after it was hit. It's not safe on the ground, it would get bombarded from orbit. Unless you can kill every enemy within 6 seconds, it will get bombarded.
StarCraft ghost cutscenes beg to differ?
No, they do not. The Ultralisks may be smaller than some of the larger units the SupCom would have, but from the SupCom trailer, they would be larger than about third or half of the tanks.
Zerg flesh is poorly built? A slow van could easily kill zerglings?
Armoured Zerg areas can easily stop bullets, for a short amount of time or for an indefinate amount of time. The Zerglings could also move faster than the Tank barrels could track them at close range, quite easily, they're agile. And a Slow moving van might hit one or two Zerglings but it would be torn to shreds within seconds. Like a cow being eaten by paranas.
If they knew were the SupCom was?
They would know exactly where the SupCom was they could detect something that large through scanner sweeps, either Terran or Protoss. Terran might not but Protoss would. They wouldn't need to get past anything else either they could kill it from orbit.
Hydralisks can tear through a foot of titanium, they could easily tear through the armour of a tank it is what they were designed to do. Their acid spines shoot for several miles. Simply put, they could get through the armour of your tanks. So could the Zerglings, and the Lurkers. And to point out Ultralisks being unable to do anything to a tank is simply retarded.
One person leading the armies.
I've seen reports of the armies being led by a single person yes, but that person could still be killed, or infected, once his armour was breached.
SupCom more powerful than SC.
Yes in General SupCom units are more powerful than SC units, but the margine of power through the units they will be up against and those of Protoss would be minimal once the larger Protoss units were brought into the battle. Also, the Zerg would more than match them in powers through the amount of Zerg there would be attacking the SupCom.
Air Defenses
The SupCom would not make enough air defense, nor deploy them in a coordinated enough effort to completely wipe out a joint Terran/Protoss/Zerg attack. Protoss weapons would bevery effective against them, Terran air units use lasers and some rather powerful missiles, and the Zerg acid spores would melt through the armour quite effectively. Say maybe 100 or so Zerg per SupCom unit, at a minimum.
Also, the point still stands, your units are outnumbered. I've seen the SupCom trailer and from that It's obvious there is no way they could build enough men. The Zerg alone would cover that entire map with an army of men severel dozen if not hundreds of layers high. Zerg which would be very capable of destroying your units. Your units are more powerful individually, though the margin slows when things like Carriers, Battle Cruisers, and the other more powerful SC units come into play, but there wont be nearly enough.
(Oh and by the way, if the SupCom destroyed a continent, he'd be blowing up his own armies if not himself...) The Zerg/Terran/Protoss forces could EASILY completely destory that planet, with your SupCom on it, I think they would have a relatively easy time killing your SupCom and his armies. Unless the SupCom's armoes will cover every sqare inch of the planet, then he'll stand no chance whatsoever.