I run a small agency (<10 people) where we don't specifically focus on outcomes, but rather "the whole picture". With the whole picture I mean, we are trying to be upfront about everything you will need down the line. Compared to the agencies you mention that focus on deliverables, they will be happy to take anything and after you deliver first thing you will realise you need a second, etc... While we would be upfront about all potential hidden costs.

The issue with our model is that we are initially seen as more expensive and have harder time finding contracts, but once we find it people want to work with us for a long time.

Email is in my profile if you want to know more.