I think chaos as a faction is not liked is cause the 2 missions dedicated to them are kinda bad or at least worse then the xenos missions, reliquary is the biggest problem here as you don't feel like your moving as your stuck in a tomb for like 5 - 15 mins and then on a bridge for 5 - 15 mins then boss time that you need to actually prep for with the CORRECT weapons and classes unlike every other mission in the game where you can bring whatever and achieve victory quite easily.

Your harming your team if you bring an assault/vanguard and not a tac/heavy cause you need ranged damage to kill it and I find plasma weapons are the best, sniper seems like a good pick but I find its just not that great.

Then you got fall of atreus that's honestly pretty good with the dreadnought and such but then you got another issue with the final part where you have to babysit a cart around like in reliquary it just drags for like 10mins espically if you need to charge it more then once but its not as bad, its a shame too cause otherwise its a very fun mission.

I believe its the 2 missions cause Vox Liberatis is fucking great if you ask me and theres no parts that really drag, you could argue that the end sction drags like fall of atreus but you can capture the alters at the same time as theres 3 then meet up for the final 1 so you can choose to split up and take 1 each then join up for the last one, it helps that it goes by ALOT faster then fall of atreus's cart section.

It has both factions as well to shake things up I'd say its more chaos then xenos but they are both in it, this is also why Vox Liberatis is my fav mission.