Not all Dig Ad agencies are created equal. In my experience, start-up dig ad companies, and ones that don't specialize in a particular industry are the ones to be the most wary of.
I have worked for a large agency that specialized in one industry that did great work. I (very briefly) worked for another start-up (25-ish employees) that took business from anyone and everyone that approached them, and after a while it blew me away that companies were still paying them at all given some of the garbage they were putting out. Even though this company had been around for 10 years, I felt like on any given day I could come in and the doors would be locked with an Out of Business sign on the door.
Look no further than the absurdly obnoxious advertising all over this site. You think anyone is clicking these ads......ever.....other than by mistake? Hell no. It's only a matter of time before the advertisers realize this and pull the plug on this site.
The entire Dig Ad industry seems unstable to me. The pretenders to contenders ratio is too high. In my honest opinion, I don't think the value is there given the cost, and if I were in an ownership position I would obviously invest in my marketing department, but would not devote a penny to digital advertising (Google Adwords Search and Display, et al).