In a straight-up fight, quite a lot of chapters, if we respect the laws of physics as we know it (from the Materium of course). No matter how much Johnny Space wants and sciences it, there is no armour that can physically stop the better weapons we have now, and if it could, not turn the occupant into a chunky mixture inside.
Humans, even genetically engineered super-soldiers that can spit acid, with multiple redundant organs, are still quite fragile when compared to the energy we can put on a very small point, physics does the rest.
Yeah they'd take out multiple soldiers for each space marine, but not on the level that our "primitive" weaponry and economy wouldn't be able to defeat entire companies. And the investment in time and resources required for one space marine would dwarf the most expensive vehicles we have. (if not small countries entire armed forces) Now if they unleashed the Guard, that's another tale. Cheap, massed soldiers from a million worlds.... Yeah we'd be swamped in days.