Out of nowhere, our users let us know that a significant part of our website had stopped working overnight. After a lot of unscheduled 3am work, we found this was because M0, M2 and M5 clusters were upgraded to a new MongoDB version without any warning or agreement from us. After talking to support, they informed us that automatic version upgrades happened to clusters without warning and these could not be disabled without upgrading to a dedicated cluster (+720% more dollar).
These MongoDB version upgrades can break your app or website overnight, and means Atlas is no longer a viable solution for us. Without bringing mLab into it - but, you know, bringing mLab into it - we never had such issues on mLab.
We did get a $10 coupon from support and they pointed us to these forums, so yeah let me know what you reckon.