What I liked most about AdRoll was that it is self-service. However, that is a double-edged sword (more on that in a minute). It also has no spend commitments or contracts. It seems like if you have a lot of time, you could get decent CPMs and conversion rates. They have also expanded their offerings to Native and Email retargeting, although I only used the display retargeting portion while I used it so I can't speak to those. There are probably a lot of people that use AdRoll and are happy with the results. However, I'm a sort of swiss-army knife digital marketer for a small business and I don't have time to constantly design, upload, monitor and tweak dozens of ads myself. So for us, it just wasn't a good fit. Now for the reasons why.
We recently discontinued using AdRoll because of several frustrations with the platform. The customer support used to be really good, until they implemented more self-service functionality. Now, talking to someone to get help is much more difficult, and the quality of service I received was very mediocre. If you want creative services, you still have to use their "request" tool, which is clunky and time-consuming to try and get a rest in. You would probably spend just as much time designing it yourself. I never had luck with the Conversion setup, and found that my conversions were always reporting incorrectly. Maybe it was user error. But I had a harder time that I have on other platforms. Also, audience targeting in my opinion is too basic. Why can I only target one page per audience? That limitation made most of my audiences ineffective and useless. So instead you are stuck targeting based on things like "engagement rate" and so forth. Perhaps my biggest gripes are that there is no in-app banner designer and the ad browser is slow and painful for me to try and navigate.