The technical aspect aside, there are a lot of "soft" factors that help: regular communication of easily consumable numbers/graphs/metrics to the client, calling out inconsistencies, etc. In other words, this is far from a fire-and-forget exercise; this can be an intense monitoring and client-engagement exercise.
The regular client engagement sometimes helps you in pinpointing cause-effect in the observations too. For ex company X might decide to advertise less of product Y in a certain state Z because of new laws there. But the team you're interacting with might not be aware of this change, or might not be cognizant of its potential impact on an advt. campaign. Regular dialogue helps here since you might observe a change in sales trends, and bring it up in a meeting - and the client team might be able to then rationalize the change. This is healthy for both parties.
I worked at an ad company. It was an absolutely standard metric to eg geo-fence ads out of a state or two for 3 months to demonstrate the impact of ads. This isn't easily externally visible, but tests like this are standard practice.
Particularly in the app install space, which is sketchy as hell once you stop buying from the top handful of vendors, buyers should be auditing by a couple million in annual spend. To get to $150m without looking hard at big chunks of their spend is just plain arrogance and/or incompetence.