Just chiming in here....
I just bought a 2015 JGC Limited a few weeks ago with the V6. I'm sorry to hear some of the issues some of you seem to be having. I know it can be frustrating to spend the kind of money these vehicles cost and not have them work the way they're supposed to.
So far, with around 800 miles, I haven't encountered anything close to what has been described here.
I've had maybe 3 instances of the transmission kind of chunking into gear on the downshift to 2, but it felt no worse than if I were to wind it out to higher RPMS, then shift - just not as smooth as a normal shift. I'd hear a faint thunk and barely feel any change in the vehicle dynamics.
I only had one shift where I could say it felt anything close to being like receiving a light bump from behind. That was after many, many shifts between 1,2 and 3 (and a mix of those up and down) while trapped for close to 30 minutes in a traffic jam on the interstate. It's never happened before and hasn't happened since.
I keep Eco on all the time, except when using the paddle shifters, since it turns off for that. Around town in stop and go traffic, I keep it in drive with Eco on. I've never had any bucking issues and haven't had the vehicle feel like it was taking off on me.
On the highway or rural highways where my speed is more constant, I generally lock it into the gear I want and only down shift if I find it needing a little boost on a hill. On the interstate, I can leave it in 8 and only shift down to 7 on some steeper hills at around 70-75. For those really steep hills where the trucks really need that truck lane, I occasionally need to step down to 6. I find that gives the tamest ride with best fuel economy.
If I let the Jeep decide, it very often wants to go down to 5 with 4 popping up if I need to speed up on a hill. I find that unnecessary, but not uncommon with automatics with anything less than a V8. I've been in 4 cylinder cars where they never stop hunting and sound like they're hitting the rev limiters on some hills.
I haven't had any issues starting off with it not being in the right gear, so it has plenty of power when I need to step on it a bit, but it shifts up pretty quickly on a tamer start. It still wants to jump down too many gears when I give it gas to overtake someone, but I've encountered that with other automatics. When it does that, I simply bump it back up manually, then go back to D when things settle.
I'm almost always in D when coming to a stop since it's hard to use the paddle shifters if I have to turn when starting off. Occasionally, when I start off quickly, but don't want to wind it out to too many RPMs, I'll shift up manually to keep from sounding like I'm flooring it.
I will say, since I started occasionally "interfering" with the automatic shifting and shifting manually the way I want it to do it, it has started to shift up much sooner when starting off when traffic starts moving quickly. It's rare it goes much above 4-5K, even under heavier acceleration. It winds out a bit further if I actually do floor it.
My point in all this is, I'm hoping this is the sign of things to come for the vehicle and maybe there is light at the end of the tunnel for those of you that are having issues. Please don't take this as gloating about not having problems - I'm just reporting that a newly purchased 2015 with close to 100 miles on it isn't having any of the issues. That could be a sign of things to come for those vehicles out in the wild already. Perhaps I'm beta testing yet another update....
...on the other hand, I hope I'm not jinxing myself.
Edited to add: I do occasionally have a little bit of a chunk happen when I only come to a full stop for an instant, then start going again. It seems the system isn't scooting into first fast enough and is still shifting when I start moving again, so it sounds like it's having a bit of a hard time meshing properly. I hear it, but I feel almost nothing - no lurching or jerking or anything.