Impact of CRM on business metrics 📈

Hello all, I was wondering if anyone here has worked on measuring the impact of CRM on business metrics - this could be anything from retention to revenue, etc. I would love to know how you went about it (perhaps a global control?) & what were the high-level results.

I have done this personally with a lot of my clients & have seen mixed results. We had tangible results for some of the apps - a 40% purchase uplift over a long period of time for a mobility app, etc. But in some cases, we saw a decline in not sending users any comms at all. 

Would love to hear your thoughts, experiences, methodology, and results.