I think newer 6 speed transmissions are very good.
Cough....cough....cough
Mine was rebuilt 3 times by GM. Google JMBX torque converter failures on the 6L80e/6L90e. You will find thousands of stories about failures
The torque converter failed, filling the transmission with glitter fairy dust
You can't remove 33% of a torque converter material, not do enough testing and not have issues associated with hydraulic deformation
The 8 speed that followed was worse and included a class action suit. The newer 10 speed is better, but is having issues and multiple TSB/recalls
The 4L60/4L60e was a much better transmission