Basically, the flaw in the summoning sickness idea comes in when you want to teleport into an enemy state (a common practice) the sick units can't make an attack since they are sick, so what happens?
If they just die, are immediately routed, or simply sit on the province as a double flag (like sieging), teleporting loses a significant amount of flavor (e.g. teleporting your monolith into an enemy state to gain control of a crucial staging point during an attack). This removes some very useful and interesting surprise attack possibilities. I like the idea of sneak attack teleports. I think the fact that your teleporting commanders will be transported sans army is a sufficient penalty. After all, you have to research Thaum-9 in order to teleport with an army (or Ench-5 for Faery Trod in forests only which is very limiting). I like the idea of doing Ritual of Returning/Teleport raids on enemy states. It gives them a reason to defend themselves more adequately instead of simply arranging themselves empty eggshells (armies on the edges, nothing in the middle). It forces opponents to consider a National Guard in addition to their expeditionary forces. You get the same effect with long range summons, but teleporting units are generally far stronger than summons because they must go alone.
The Sphinx-teleport is an obvious weakness. The Sphinx is nigh indestructible early game and should not be allowed to teleport. It is a special exception not the rule. Other pretenders (with the possible exception of the Monolith which is closer to the black side of the gray area IMO) can't usually take on a national army early game. Worrying about early teleports forces you to Patrol provinces instead of just sitting around and ensure that you have a backup base. Besides, early teleport attacks require scouting. He has to find you first and develop Teleport capability (not hard, but I tend to prefer Difficult research as it makes a better game for mercs and nationals) and make sure you don't have the strength to beat him down. Losing a pretender early in the game when priests are low can cost you dearly especially with a pretender that has a diversified magical portfolio. Stealth or Glamour units would be a good way to lure your opponent's pretender into a deadly trap. Manticores that find themselves surrounded by blessed Armor Piercing Vans/Bane Spiders (or just a couple of Revenants with a Level 1 Decay spell) won't be happy with the odds of victory. I've had my pretender decayed by indeps before--which is rare but sucks especially early-game, so I try to stay away from necromancers. Shadow Bolts/Magic Duel can also be dangerous and higher level magics can easily overwhelm a lonely pretender or at least keep him away on the off chance he'll get killed or feebleminded. You kill the big dudes with fatigue--Stellar Cascades is great for killing pretender SCs esp those w/o reinvig. Sleep (because of its MRN feature is less useful), but it's relatively easy to cast and small number of casters have a fair chance of getting the big guy to 100+ fatigue and subsequently trouncing him. Of course this is all provided your enemy is not a giant rock which is where the trouble comes in
