Drew is a great writer.
As a side note there's something interesting I think some people should know concerning ME2's story (saying this because lots of people do know that many things in ME2 are just not clear or well explained in the game, in relation to ME1). There's a good number of posts over at the BioWare Social forums where you can hear so called "unused" dialog by the developers, but recorded nonetheless and at some point in time was considered to be implemented in the game, but was for some reason(s) cut out eventually.
A lot of that unused dialog ends up often explaining in greater details (and by different characters when compared to what we have in the final game) many aspects of the ME2 "links" to ME1 such as in relation to the Reapers, the Collectors and why they do what they do (getting samples from various races, making a "Terminator" Human-Reaper hybrid, etc). I've heard a lot of such unused dialog myself and I must say that whatever anyone may think regarding the actual writer(s) of ME2 (be it Drew, Walters or anyone else whom might have been providing assistance) that if they do listen to that dialog then they'll soon realize that the responsible people for mixing up the story are the ones editing the game and making the final decisions over at BioWare, as to what stays in and what gets cut out of it, or what gets replaced or changed. The actual writers, I'm sure, knew what they were writing and they surely had a clear, detailed picture of the whole ME franchise in their mind from the start.
At some point as far as ME2's story-telling is concerned (and most likely to follow suit with ME3, unfortunately), the actual existing useful and detailed explanations were cut, shrunk, and/or modified in preference for... let's say a more "subtle" approach, one that in the end only left many of us scratching our heads and slowing stepping backwards away from the game's credits thinking "ok, what the hell did just happened?". Those responsible for that were not the writers (which I do honestly believe), contrarily to what many may still think to this day. But that's normal, because out of the hundreds of thousands (if not millions) of fans of the ME franchise I doubt that even a tenth of them actually known about any of that unused dialog, and what it implies for story-telling (and the quality of it) for the games (in general) that are being affected by cut out/replaced/diminished dialog later on.
It's a bit of a story-telling tragedy, in my opinion. If I was a writer myself I'd make sure not to work for a company that knowingly cuts away the stuff that I would have written for the readers and players alike, with enough details for them to actually be able to understand what the hell I was trying to explain in the story in the first place.