We did the same (but to mLab) -- we had a terrible experience at Compose
Would you be open to sharing some details about your experience there? I'm building a competing service and want to learn how I can make it great. If you're not comfortable sharing publically, my email is in my profile.
We also left compose but left to move to mLab. When Compose was still MongoHQ, we were really impressed by their support. In the last few years, we became incredibly unsatisfied. mLab has been awesome and responsive. I hope the move to Atlas doesn't cause their support to change.
What didn't you like about the support you got from compose? I'm working on standing up a competing service and would like to learn from your experiences with them.