A process in a namespace is running just like another process being managed by your kernel. Based on how you set up networking, you may face an extra hop to get packets. I don't know what other scalability issues will be there, it's literally a process running similar to other processors.

Can you shed light on some of these, maybe I haven't encountered these in my day to day ? (Please note I am not talking about containers running in VMs, which apparently Docker does now).