Synthetics do have properties that far exceed a dino oil in friction, viscosity, temperature tolerance, and film strength measurements.
An auto trans application, though, typically doesn't suffer from point friction loading issues or heat breakdown from single origin exposure (turbocharger housings, piston crowns).
Trans fluid's biggest test is in the torque converter. That is the spot that gives ATF nightmares. Kinetic shearing causes the excess heat we all see on the trans temp gauge, and most of it occurs in the TC unless your trans is slipping. Properly operating trannys only subject the fluid to momentary band slipping during shift events. The gearface friction in a modern trans does not approach the recommended limits of the lube.
Trans fluid pulls a double duty as a straight hydraulic oil AND a gear lube AND a wet clutch style fluid. It must perform all of these duties while being polluted with byproducts from the other functions it does.
So far, I've been a believer in using the fluid spec specifically called out by the mfr. But I have been using a typical brand name conventional product.
I'm as interested as you are in hearing thoughts here.