Ok, I'll admit my knowledge of Imperium Naval forces is somewhat limted, but I do know that the Ultramarines can field the most large vessels of any Space Marine chapters, and since they answered the challenge, and have the most experiance fighting a hive mind, lets go with that.
Now, the Borg have been shown to defeat huge fleets of Starships in both TNG and First Contact. Of course, in TNG at Wolf 395 they had a huge tactical advantage, but at the Battle of Sector 01 in First Contact the Federation had every advantage in terms of new technology and new ship designs and new tactics, but the Borg still blew away dozens of Starships, though they did take extensive damage.
In traditional combat space naval combat, a Borg Cube is nigh invincible. Even if a large flotilla of Imperial ships could pound their way through the shield and start tearing the hull apart, the cube could probably destroy any ship in a 1-4 shots (thats about avergae from a cube) giving the Imperium high attrition, in addition to being able to repair itself, which could outrun the damage as the Marines loose ships
However, if we assume a Nova Cannon can do enough damage to cripple or lead to the chain reaction destruction of a cube, it become a differant ball game, as the drones use their vastly superior Star Trek transporter technology to engauge the Space Marines on the planet and/or their ships (The average cube has over 10,000 drones as crew)
Obviously, the borg can adapt to a lot of the space marines weapons, like lasweapons, Plasma weapons, flamers and melta tech. But bolters and melee weapons like Chainswords or Power claws are not energy based, and may not be adaptable (Those borg didn't seem to have any way of adapting to Jean Luc Picard's tommygun on the holodeck) The question then becomes how melee combat between Borg and Space marines go. Can their enhanced biology resist nanoprobes and assimilation is another good question.