Without doubt, the single biggest problem with the 6HP26 is faulty solenoids. These cause the vast majority of issues with this transmission. The other main issues are:

• Transmission refuses to shift higher than 3rd gear (E-clutch slipping due to hydraulic leakage caused by wear in lead-free rear stator bush)
  
• Clunky 2-1 rollout shifts (Latest mechatronic firmware updates not performed)
  
• Clutch packs destroy themselves following fluid change and adaption reset (ECU cannot find new adaptions in time to prevent clutch plates burning up, due to change in friction coefficient)
  
• Complete loss of drive (Failure of output shaft spline through fretting corrosion due to lack of lubrication at factory)

Mechanically though, I would say it is a more robust transmission than the 5HP24. Having said that, Audi put the 5HP24A behind their 6.0 litre W12 engine and even the 353kW/560Nm Audi RS6 Plus, so the 5-speed running gear is certainly capable of handling more power/torque than the 4.6iS E53 delivers.

Phil