The power and effectiveness of digital targeting is often overblown for small businesses.
Many of my small business owner clients hear anecdotes where a business was able to use targeting to create a veritable "money machine". The problem is these scnerios are fleetingly rare, complete bullshit or only apply to a specific niche.
The anecdote often glazes over the thousands of wasted dollars spent trying to perfect the model and a hundred other details that factored into the success.
The good thing for most is that digital advertising is no less effective than traditional advertising. Even if there's little to no targeting, or no tracking in place it's not any worse than tv/radio/print.
I can count on one hand the number of clients in my 20 years of web work that have created a model where they dump money in the top, and are able to extract more out the bottom. In all cases it didn't last more than a couple years at most.
For any small business considering hiring an ad agency that claims to have mastered this model ask yourself this: Why aren't they doing it themselves?