I've been meaning to write about this as well, but before I can I have to compile real data. So I don't have that for you yet, but I'll pop in anyway.
The BIGGEST problem I see with Express is business owners think they can do it themselves! Now you may laugh at that because that is why it was designed, but the fact of the matter, they are not qualified and Google is not good enough to make the best decisions for them which it is trying to do with their "smart" algorithms.
Let me give you an example.
Client 1 was spending about $13,500 a month on Express came to me and said he was losing his shirt. Told me he got on average 1 1/2 leads a week. As an attorney, he doesn't need many, but he needed more so that he could be pickier about the clients he took on. And at $13,500 he wasn't getting enough to justify the cost. (He is not a personal injury attorney.)
So I set up an adwords campaign for him. It took a month or two to "perfect" it as best I could on a small budget, but now he gets 3 or 4 leads a week on ONLY a $1,500/mo budget. That's huge! On top of that, I did SEO and he now gets that many leads from SEO as well.
Client 2 has a much smaller budget so she has much less to lose. She is spending $130/mo. I have not followed the details on this because I was called into do optimize her new site for SEO is all. But I do have access to her Google Analytics where I can see clicks from her ad campaigns. She spends roughly $10 a month on keywords that matter. $120 is a complete waste. Why? Because Google has chosen to show her ad for terms that might appear to be relevant to the ignorant and bring in an entirely wrong audience. And she doesn't know better or how to even fix it. I've not had my hands in Express to know if you have this ability, but I gave her a list of keywords to add to a negative keyword list. If Express provides that, that's great, but you're back at the problem of these SMBs trusting systems to do it for them, and the bigger problem that they don't really know what's best, how to fix what's wrong, or to even know how to tell when something isn't right.