They're killing performance (one of the main reasons to use containers) and adding a massive extra layer of management if they're running containers on EC2. I suspect Netflix are too wedded to AWS (which is weird as Amazon is their biggest threat) but Triton or Red Shift (both of which actually isolate containers using SmartOS and SELinux respectively) make way more sense for other people who want to use the blazing…
I'm not running that combination myself so I wouldn't really know, but I'm not aware of problems with that specific combination or can think of anything obvious.