In our production notes, we state:

With the WiredTiger storage engine, use of XFS is strongly recommended to avoid performance issues that may occur when using EXT4 with WiredTiger.

As this would not need to apply to arbiters, perhaps we should restate as:

With the WiredTiger storage engine, use of XFS is strongly recommended for data bearing nodes to avoid performance issues that may occur when using EXT4 with WiredTiger.