Overlapping audiences. Should you be concerned?

Having a huge percentage of overlapping audiences across your ad sets can lead to hindered delivery of your ads if they are under the same campaign. This is because only the better performing ad set will be picked to enter the ad auction process to prevent your ads from bidding against one another. 

Another reason to consolidate your ad sets would be to prevent spreading your campaign budget too thin across too many ad sets. When ads go live, they will begin the learning phase where the delivery system is learning about the best people and placements to serve the ad. 

Ad sets will need to generate at least 50 optimisation events in 7 days to exit the learning phase. These ‘optimisation events’ depend on what the ad campaign is optimising for - it can be purchases, add-to-baskets, landing page views, app installs etc. 

By having too many ad sets, you run the risk of not being able to generate sufficient events in order for the ad set to exit the learning phase and have its performance (like costs & results) stabilise.