Cleon
Legend
A quick spot-check indicated most had none (dretch, imp, quasit, lantern archon). The best I found was the hound archon (SR 16, CR 4).
Fiendish and celestial creatures have SR = HD + 5, so most summoned monsters around that level would have SR less than 10.
I'm not seeing the turn resistance based on SR as a huge deterrent to the cunnequine.
Well Half-Fiends have SR+10 which ain't to be sneered at.
I think I'd better consider some examples to see how much it makes a difference.
Fiendish Huge Viper (CR 4, HD 6, SR 11 => TR +5 = Turns as 11HD creature)
Fiendish Large Spider (CR 3, HD 4, SR 9 => TR +4 = Turns as 8HD creature)
Half-Fiend Ogre (CR 4, HD 4, SR 14 => TR +7 = Turns as 11HD creature)
With a 1d20+7 turn check a cunnequin would average about cleric's level +2, so it would typically turn creatures up to 8th level (4 HD +2 for its turn bonus +2 for its typical turn check).
That's enough to turn a Fiendish Large Spider roughly 50% of the time, but it'd never be able to turn a Fiendish Huge Viper or a Half-Fiend Ogre since it would have to roll "cleric's level +5" on its turn check and the maximum the table allows is "cleric's level +4".
I suppose those numbers work out alright, so we may as well put in the SR adjustment.
I'd still prefer to have "banished" instead of "destroyed" though, but then I prefer the oldschool approach to turning extranplanar creature.