The campaign can be played with all the six factions present -- like in the expansions to the first Dawn of War. It differs somewhat for each race but mostly you play the same missions in the same order. I of course continued with Space Marines.
I wanted to see how the story ends from their viewpoint, even though it is necessarily not canon -- Warhammer 40k games seem to have bit of a habit going for the grim and dark. Like, for instance, the Kaurava system campaign lead by Gabriel Angelos in Soulstorm apparently ended up in a complete catastrophe for the Blood Ravens with a large part of the chapter killed in action.
The story ended with Blood Ravens securing the system and Gabriel Angelos becoming their new chapter master after the traitorous old one had been killed. I recall reading years ago some short Blood Ravens lore article that claimed Angelos to be the chapter master. It is considered conflicting lore, however, as he had only appeared as the captain of the 3rd company until the end of this expansion. I wonder how canon the Space Marines ending is this time -- did Angelos really become the chapter's leader. I guess the only way to know will be when/if a third Dawn of War game ever happens.
Retribution's gameplay -- at least in the campaign -- takes a step back to the first game. By gathering resources and capturing locations, you can field more units in addition to the four heroes you already have. I was not very keen on this change. Micro-managing 10+ units kind of breaks the tight tactical action that was previously going on. They should have at least made the squads be joinable by the heroes.
The skill system was streamlined a bit for Retribution by removing the no-trait points. A smart decision, I think, though many of traits do seem to fit even less under the category they are in now -- strength tree's traits improving ranged combat etc.
All in all, I did not enjoy Retribution as much as I did Chaos Rising. The technical issues added to that. I expected the same problems continue as the engine for the expansion is the same. However, the old blackouts and hard drive torture were not as much of a thing -- I even played on higher than medium settings. Instead, there were these straight-up crashes that were always preceded by game sound (apart from music) disappearing a minute before. You can save the game but of course the crashes mostly happened during boss battles and such when saving is evidently not a thing you can do. Replaying the maps was quite the annoyance to say the least.
Also, one small thing I find amusing are the titles of the Warhammer games on Steam. Games Workshop are known for their zeal of guarding their IPs aggressively and this expansion's full name kind of gives it out: Warhammer® 40,000™: Dawn of War® II - Retribution™. I mean, even the Roman numeral would probably have a trademark icon on it if GW were allowed to do so. I do not think it is necessary to have the symbols on a game's title on the Steam client, especially if the name is long enough already.