I was advised over a decade ago against transmission flushing (by a mechanic). He said that often they clean it “too well” but did not elaborate.
I will add that the only time I have ever experienced transmission failure (on a newly purchased care under 60,000 miles) was following a transmission treatment. The car was still under warranty and the engine ran in great shape until the frame was compromised in a rear-end collision at over 100,000 miles.
A buddy of mine had his Chrysler minivan's transmission die right after 60K. He had not changed the fluid. He's not sure if it would have made a difference. He skipped this issue for his next vehicle by buying a Honda.