I had a so so experience with Greenback. I paid them I think $1,500 to do a streamlined resident application for me (with a half penalty I might add) and I ended up not using it because I got better advice that I didn't need to do streamlined because I didn't owe any tax. They were thorough with the returns though. I suppose it depends what sort of a position you want to take on issues that aren't clear. I chose someone who took a different stance on some thing which we was prepared to defend if questioned. The thing about Greenback is I think they just farm the work out other accountants - I doubt they have any people on staff Therefore it just depends which accountant you get.
I believe greenback has a small staff for managing the website/accountants. The accountants themselves are all qualified CPAs, generally with significant foreign tax experience. I believe they operate as independent consultants, so it is a way of simply connecting you with someone. So I suppose it does depend a lot on who you get. My GB accountant fortunately was excellent, really saved my bacon in a complex tax situation. Her background was in foreign corporate tax returns which was perfect for me, and she has saved me a lot of money in the end.
Having said that, if there is anything unclear, I will usually double check with taxes4expats to get a second opinion. I think it makes sense simply due to the sheer complexity of US tax law in certain situations. It can also be worth paying a US tax lawyer that specializes in foreign taxes for a legal opinion on something, if you are really concerned.
I wonder also if you could ask for another accountant if you're not happy with the one assigned.
