That is right. Unicast is much better than broadcast.
However it depends who's ArtNet boxes you're using. Our Ethernet-DMX8 can cope with up to 100 universes in broadcast (actually never done a test above).
But as unicast is possible, it is always better, because many devices don't have that processing power to process all the data, to check if it is a universe they have to output.

Grtzzzz