We're in the process of migrating our legacy stuff to mongodb right now.
If you want a high-availability database solution that "just works" and is open then you don't have many other options.
(Postgress and Mysql don't have automatic failover and recovery.)
MySQL Group Replication isn‘t automatic in all cases, like starting after all nodes failed, and it has some limitations, but it is built-in.
Not saying that MongoDB ReplicaSet is bad, has been working very well for us AFAICT.
Mongodb is entirely automatic, you just delete bad servers and reprovision them automatically and everything "just works" with no interruption.
In the same sense that the GPL is more open than the MIT license; more viral requirements for openness are generally a good thing. I don't want Amazon and the ilk deploying hosted Mongodb clusters.
Open source for me is a hedge against risks. (Especially political and/or legal ones.) Mongodb's current license resolves that issue for me perfectly.