Hence the speculation that that Tyrannid swarm is not that produced by a single player, see? Else it really might well be totally ridiculous if a few Marines can wipe it.
But yes, they've taken their inspiration from the novels mostly, where Marines are very very dangerous indeed; more like Jedi than just elite soldiers. It's certainly nothing close to the expendable units of thei first game.
Except for the Tyrannids. In DOWII there is a veterancy system where your units improve as they score kills, so conserving units is a big deal- but the basic Tyrannid units do NOT do this, so they are absolutely expenable.