You can do any of the non story focused missions after ME2 ends, including all of the DLC as well as the standard side quests that ship with the game.
HK-47's description as it being a 'cash-grab' are pretty spot on. As I said, I'd describe it as 'half-arsed'. Where Kasumi, Overlord, and Shadow Broker developed an identity, and were complete with memorable moments and characters (to varying degrees), Arrival is largely devoid of any of this.
All the environments, except for the final vista, are uninteresting and very in asset use. The level design is by-the-books, even for Mass Effect 2 standard. There's an almost exclusive focus on combat, and even that is unexciting thank's to reused assets, no unique enemies, and bland arena design. The story does have some good moments and a decent arc, but is still thin on developments, with certain dialogue lines and a particular confrontation coming across as quite unimpressive and forced. The cutscenes were also animated poorly.
The kind of polish and effort that went into even shorter stuff like Kasumi just isn't here. As the final hurrah to Mass Effect 2, and a piece of bridging content, it is very disappointing. I'm a pretty big Mass Effect fan, and I would rank this down the lower end of Mass Effect 2's mission quality.