You can't make a _modern_ harbor quay out of it, one that is strong enough for the super large container cranes. The main problem is of survivorship bias: all the flimsy Roman harbors have disappeared so yes, only the most sturdily constructed ones remain. They would still not last under the "modern" load of 40+ ton vehicles.
Are you sure about that? The Romans built a lot of harbours. We don't really have any data on the compressive or tensile strength of Roman concrete so it's hard to say for sure.