In my opinion also, the the back over the barrel / Reflex suppressors do not serve a use as well as modern high quality front mounting suppressors.
For both optimum sound and flash suppression, the baffles situated in front of the muzzle achieve more performance than the expansion tube / chamber rearwards. You also do not see the best flash suppression from a suppressor that adds little length.
Just looking at the sound suppression, modern front mounting suppressors can be as good or even better than some of the back over the barrel suppressors.
So in essence a lot of that back over the barrel length and weight is wasted space and weight, if not utilized in a more complex form for a lower back pressure design.
In which case the suppressor gets heavier and more complex.
I agree, the AEM5 is an awesome can. As good as any 223 can I have ever seen and it doesn't add the length of a "full frontal " suppressor. I swap mine between four different ARs and its repeatable and accurate to 600 yards on barrels from 16 -20". Stays put on my rifles as well, but I ensure at least 6 thread engagements on my rifles.
I think barrel over comes down to the users end requirement, much like weight, FA rating, etc.
I have been using the AEM5 (.30 cal) on 5.56, 6.5CM, .308, and .300 win mag and this is usually the quietest can at the range. Everyone that has heard it loves the " tone" of this can. I am a fan. I have a SAS Covert suppressor setup in Jail and will give you that update when I get the approval. This SAS can is a total over the barrel setup.
I also run an AEM5 on my MK12 Mod 0 clone. Easily the quietest can I've used on a 5.56 gun. POI shift is minimal and repeatable. Ron Allen is a great guy to chat with and he will do anything he can to help you out. Keep in mind this was a result of a clone and dictated exactly which can I purchased, but I'm very happy with it.
This is the extent of my experience with reflex cans though. As others have stated, modern tech and materials probably make them obsolete outside of cloning. The MK13 Mod 5, MK11, and M110 all also use reflex cans but .mil isn't always concerned with absolute sound suppression as much as signature reduction overall.