I think a vertical list is much easier to parse visually. Typically we expect this list to be of length one, occasionally it will be two or three if replication is used.

Many clusters have long hostnames and if they are in a list the part that is different lines up well, e.g.:

    mycluster.node.03.some.long.suffix
    mycluster.node.04.some.long.suffix
    mycluster.node.33.some.long.suffix

If we do comma-separated, it's likely to get wrapped anyways, we just have less control over where it decides to wrap.