The problem with using the Borg for Nemesis
rather than First Contact
is that by the time Nemesis
was in theaters, the Borg had already been used heavily on Voyager
. People were sick of them.
Romulans, by contact, were still something of a blank slate and thus more appetizing to a movie audience. The problem is that the Romulans ended up being depicted not as the villains of Nemesis
but as willing participants in Shinzon's plans for genocide until Donatra decided to stand up to him.