Unity looks great, but has way too much copy-pasting of assets. You see the same buildings repeat again and again, with the same interiors. Compare this with GTAV, where at least 80% of the buildings are unique (though only 1% of them have interiors). In addition, GTAV feels more vibrant and colorful compared to ACU. (I haven't played FC4 yet but I assume that one is twice as colorful as GTAV)
Certain things reveal the last-gen roots of GTAV, though. The majority of windows are flat low-res textures, and there are only a few buildings with fake-interiors. The realtime envmapping reflections on almost every window somewhat makes up for it, though, at least compared to games that use generic cubemaps with no relation to the actual world geometry. (Looking at you, Watch Dogs)