You can tighten containers but at the end of the day they are running as native processes on the same kernel. Any vulnerability and game is over. VM offers an easier (maybe heavy) mental model of security. Between guests and between guest and host.
A jail breakout, it being from a process namespace or a VM, is always a security-risk - whatever it's breaking out of. Both are sensitive to this. VM's are maybe a bit more mature and handle some stuff on hardware - but given the recent Intel oops thingies - I wouldn't rely on that too much...
"Containers are less secure" is just FUD. That VM's or containers alike are running on the same CPU is currently a much more real threat.
