Few months back me and my team work in mongoDB 3 stage Production Upgrade with Replicaset as follow the Path.
1.) DB and Backup DB upgraded from v3.0.4 to v3.0.15
2.) DB and Backup DB upgraded from v3.0.15 to 3.2.16
3.) DB and Backup DB upgraded from v3.2.16 to 3.4.7
Now currently running version 3.4.7 and facing some issue in this version and its known bug later to know after upgrade.
1.) The Bug is when some one object(Table/Index) is created in Primary Node and same resync on all the nodes.
2.) After resync again drop the same object from Primary Node so dependent datafile is also drop from filesystem. But resync process doesn’t work i.e object also deleted from another node but the physical datafile not deleted from secondary node and consumes lots of space.
Resolution Available on : v3.4.11 version.
So due to some down time issue from application side we fix this issue for temporary basis.
For more detail about this bug: Click Here