<bAlwaysHostile> is actually a bit of a misnomer, as units with the tag don't seem to be hostile as far as rival cities are concerned. I was expecting them to attack units in cities the same way that HN units do, but to my surprise they instead entered the city peacefully. I rather like this, as it means my Dragon Fanatics can still spread their religion in rival cities but you can still attack rival dragon fanatics to prevent them from reaching your cities to do so. (In case it is relevant, I'll note that these units have the <bRivalTerritory>1 tag too.)
It seems to me like bAlwaysHostile units should not be able to capture super forts from players whose cities they cannot capture. Note that they still can capture cities while at war, just not while at peace.