One of the downsides to rootless, in my opinion, is the networking support is limited, rootless containers can talk to the host but they cannot talk to each other unless in a pod, if there in a pod you have a single network namespace and therefore you cannot duplicate ports.
If you spin up multiple rootless containers and look at the assigned IP addresses you’ll notice that they are the same, you should also notice multiple instance of the slirp4netns process one per network namespace. Spent a lot of time scratching my head about how this worked when first looking at it.