I changed my tranny at around 200k but, it had been abused by previous owner through improper towing.
The interesting part is, I put a tranny in that had 416K on the clock.
I really did question my decision to put in a tranny with such high mileage but I had it, it was free and fluids did not appear burnt.
So far, pleased to say it has performed flawlessly for the last 25 k since install.
Fluid was changed at the time but no flushing or anything like that.
Essentially, tranny's are well designed with no known issues but they will not stand a lot of abuse from overheating or improper towing techniques eg: allowing o/d to engage while towing with anything but the lightest loads.
Assin tranny's were also used in some Jeeps