I've had Transynd in my '01 Lumina's 4T65e for the past 30K or so. This particular transmission didn't shift well when I 'upgraded' to DexVI (the 1-2 up-shift started a nasty slip & grab behavior at full throttle). Did a complete flush with Transynd and it solved the problem.
Transynd is a synthetic atf that meets DexIIIG specs, along with a laundry-list of industrial specs... and should work fine in any application calling for that.
As for whether Transynd or DexVI is 'better' in the Allison transmissions in those GM trucks- I dunno. From what I've read, there's very little in the way of 'proof' or solid info that's readily available to us mere consumers (lots of claims, lots of anecdotal evidence... very little in the way of data). The choice of OEM fluids in that particular transmission is based more on corporate politics than any distinct advantage. Personally I doubt you'd notice any real difference between the two.
In non-Allison applications calling for DexVI, I have no doubt that Transynd would work. But for how long? And under what conditions? And would it cause warranty issues? Transynd is a weird looking ATF once it gets some miles on it- turns a strange yellow color with a reddish tint... it would DEFINITELY be noticed by the tech if there was a warranty issue. IMO it would be analogous to putting a fluid like Amsoil, Maxlife, etc. into an application that wasn't designed for it (although one would assume that Amsoil and Valvoline have done some testing for automotive applications... I dunno if we can assume that with Transynd, with the exception of GM pickups. It would work- but you'd be taking your chances.
I've seen no reason to believe that Transynd is a "better" fluid than DexVI in any way. My own results tell me that the friction properties of Transynd are somewhat different than those of DexVI- and said friction properties of Transynd work better in my 230,000-mile 4T65e. But I wouldn't bet that everybody else would have the same experience. If a transmission was designed with DexVI in mind, the friction properties of DexVI would probably be more suitable than those of Transynd.