I'm a contributor to the Sandstorm.io project, and Mongo continues to present a bit of a pickle for us. We can arguably write our way out of one upgrade, but upgrading to a later Mongo just punts the issue again. We'd much rather just leave Mongo behind.

What kind of problems does mongo cause for sandstorm that postgres does not?

The core of the issue is Mongo does not seem intended to be upgraded reliably without intervention. Sandstorm is running on thousands of servers where the admins aren't equipped to handle Mongo upgrade issues, as well as within some Sandstorm apps which also use Mongo inside containers not intended to be user servicable.

One of the issues we hit is here: https://github.com/meteor/meteor/issues/11666 in which if you happened to have a Mongo database over eight years old (many Sandstorm servers have been deployed for that long!), you needed manual intervention to correct it, even if you had done intermediate version updates in between.

Meteor patched around this issue... but after dropping support for several releases of Mongo. So we essentially need to build our own automation which understands and can export old Mongo databases, and then import new Mongo databases, while shipping a Meteor app that can only run on one or the other, which has to auto-update smoothly, and recover from failure like if there isn't hard drive space to handle the process.

And then we also need to implement that within app sandboxes which can also arbitrarily terminate so that also has to recover well and we need to ship the logic to do this with every Mongo-backed app package until the end of time.