the problem with port mangling is that your application starts running on random ports, so in addition to requiring discovery for IP addresses, you now also have to do discovery for ports, which pretty much requires custom code and infrastructure linked into your binaries (how do you convince nginx/redis/... to use your lookup service for ports?)

And ports are different between different replicas of your service, since they're chosen at random during scheduling.

It also makes ACLs and QoS harder to define for the network, since you don't have a clean network identity (e.g IP Address) for each application.