A .38 Super load will fit the cylinder of my F/A .357 or GP100, but will not enter the cylinder of my S&W model 10 (good). The brass on .38 Super is generally very thick at the case mouth.
I have never attempted to fire .38 Super from my .357 revolvers, but I have fired a few .38 Supers in a T/C Contender with an MGM .357 magnum barrel. Shot just fine, but the cases needed help getting extracted.
I don't like the fact that the bearing surface of a .38 Super bullet is unsupported after it leaves the shorter Super brass and before it is supported by the .357 throat. The bullet has a chance to yaw in that gap. With plated or lead bullets, any misalignment as it hits the shoulder at the end of the chamber will scrape the bullet.