I'm going to post my experience on here in a couple of places in hopes that my info helps someone. Sorry in advance for overposting.
I've got a 2010 Flex EB and have had transmission trouble almost since it was new. The local delaership has given up trying to fix it and I'm about to get out of my lease early (paying the penalty) just to get rid of this awful vehicle. I'm so tired of it I can't tell you. It's sad because the Flex had so much potential. I really loved it when I bought it.
Symptom: It feels like an engine problem because power is drastically reduced and surges contuniuously when trying to accelerate. Sometimes it will take off with full power, other times it will give you no more than 10% power, regardless of throttle position. When it does get full power, it does for only a second or two before the surging starts. The engine will growl and you can hear what sounds like boost being vented. I've even heard the engine knock in short bursts sometimes. This happens at all throttle positions under normal driving conditions. It does it slightly less when cold, but pretty much does it all the time.
I've had it to the dealership for this issue no fewer than 8 times. The first 2-3 I was told it was driving normally and experiencing turbo lag. Then they finally admitted they felt a problem. They reflashed the ECU, replaced the throttle body, replaced spark plugs and a coil pack or two. These things were done one at a time during each visit, each time saying they fixed it. The last two trips for service were using their "flight recorder" that plugs into the CPU port to record data. The engine data looked fine, so I did it again to have a look at the transmission. They said they found some issues and replaced 3 "plates" in the transmission - they had the Flex for almost two weeks to do this. When I got it back they again said they fixed it. Again it started acting up within minutes of leaving the dealership. I brought it back again after about a month of worsening symptoms and they finally told me they couldn't figure it out - they gave up.
That was several months ago. I've been driving it this way ever since, always wondering if I'm going to get stranded somewhere. It's terrible. It does it constantly and at times is so bad it's barely driveable.
This, along with other issues I'm haivng with the vehicle helped us to decide just to break our lease, paying about $1500 in penalties just to get away from the car. The other issues are: grinding brakes, front shocks that feel like they're blown out (ever since about 10K miles), clunks and whirring coming from the CV joints (since about 20K miles), engine knocking on startup that started at about 20k miles and has gotten progressively worse - now 40k miles and the knocking lasts about 4 seconds on a cold start (dealer says CND), and general body creaks and rattles.
I hope the symptoms and attempted repair details noted will somehow help someone else with similar problems as they attempt to fix the issue. I had trouble finding data from others when this all started, so I promised myself I would post my experience.
I've also got a '11 F-150 EB, but it has none of these issues. It's got about 15K miles on it and so far so good. Sure hope it stays that way.