Flag waving missions depend on the crew - it’s they who do the waving! From local football matches through spending in bars to painting orphanages and hosting CTPs, that needs people. I’m not sure an unmanned platform being controlled from afar would have the same effect. In fact it might be seen as quite sinister! Also, there are no real rules as yet around third party port visits, or anything else much, for USVs. I’m not sure, for example, that they would enjoy the flag State immunity that manned warships do. To be more prosaic, how would they comply with pilotage, or pollution control requirements? The next few years are going to be full of interesting developments!