United Orks, at least a chance to turn them back but with extreme casualties and be very weak, almost too much to come back. For Tyranids, no, the full force would simply be too much to take even if all the marine could concentrate and attack the 'nids would still have fore numbers than could be dealt with. An 'army' of carnifex's would be a headache for any force the marines have and dont even get started on the huge bio-monstrosities that they could bring...