Nids are varied, probably one of the most varied factions in 40k.
But they also utterly lack any semblance of personality, making them, in my opinion, pretty boring to fight in the long run. At least Necrons higher-ranking individuals can be super fun (anyone should read "The Infinte and the Divine" btw), but Tyranids? Just... bugs.
That being said, if they want to implement the same kind of multiplayer the first game had, they pretty much have to include at least one other enemy faction (unless we're all playing as 'nid Warriors or something, which I guess could work).