If you have extra ad spend to work with...
UTM Tracking - You might choose to use UTM tracking, which can be setup in the settings of an account, campaign, or ad group.
You will see consistent correlation.
In some instances, yes. But the pros far outweigh the cons. Here are some reasons why you want to run branded campaign projects and how it can get you to the top SERP spot.
Doing this causes the algorithm to not be able to spend the full budget, and ultimately, causes us to fall behind spend and lose valuable time in the learning process, too.