I did all my tuning on the street with a AEM digiatal WB. have had no issues with mine it works great falls right in line with the narrow band o2's at cruise or light load of WOT. I also know ppl who use them on other cars with no prob.
as for a "LOADED DYNO" and mustang dyno is a eddie current dyno and can create load VS speed anywhere in the range of driving. It can also create more load than what you will ever see on the street with can run you higher in the Base fuel table and if your tuner didn't change the tables correctly at the lower load cells you could have a rich/lean problem there.
just my experience/2 cents
If I was going by what the aem gauge reads at wot , I would be extremely rich and the car would be rolling coal big time . There are no issues with my tune . As stated previously , on the dyno , my a/f ratio is 11.7 . On my aem it pegs out at 10 at wot . So its obviously a problem with the gauge . I have read on multiple other forums of people having the same issues with the aem gauges . I will be getting something different to solve this problem .