In a so-called "pitched battle", I think there is little doubt that a full Legion (led by a Primarch) would triumph.
However, in an invasion of Commorragh, the advantage is played into the Dark Eldar's hands. The dimensions of the Dark City are described as:
Originally Posted by Codex: Dark Eldar
In the depths of the web way lies Commorragh, the lair of the Dark Eldar; called the Dark City by those who fear to speak its name. Commorragh is no mere metropolis, for it is to the largest of Imperial Hives as a soaring mountain is to a mound of termites. Its dimensions would be considered impossible if they could be read by conventional means. If anything, Commorragh is more like a vast collection of satellite realms and cities linked by uncounted portals and hidden pathways. Viewed from one perspective, Commorragh is a loose collection of far flung nodes spread throughout the arteries of the web way like a malevolent virus. Its clustered concentrations are in reality scattered across the galaxy, thousands of light-years apart in places. Yet these locations are linked together by shimmering dimensional short-cuts. From within the web way’s confines, the immense distances between each sub-realm can be crossed with a single step. Commorragh appears within the web way as a composite entity of impossible scale, a shimmering, contradictory realm the dimensions of which pluck at the sanity of those who approach it. Thousands of ships dock each day within its out flung spines, for the Dark Eldar are far more numerous than even their Craftworld kin suspect.
Any full-scale invasion would require the ability to outmanoeuvre and out-think the leaders of the Dark City. It seems quite a simple process for Vect (or other Archons) to seal (or otherwise hide) the interconnecting "portals and hidden pathways" that necessitate travel between areas of the Dark City. Combine that with the knowledge that the Dark Kin hold concerning the webway itself would put the Space Marine Legion at a distinct disadvantage, perhaps even completely unable to access the Dark City. In many ways Commorragh is unconquerable.
However, as someone else has already pointed out, if we're talking about a hypothetical invasion occurring during the Great Crusade (when Commorragh was not as developed) the advantage would be less clear.
And not even the whole chapter. Wasn't that just, like, a hundred marines?
It should be noted that the invasion was facilitated by Vect with the express intention of wiping out (or dethroning) the ancient noble houses. Everything they achieved is thus likely down to Vect.