When the data type of the shard key is BinData, log messages regarding chunk limits and split keys are not very informative since BinData keys are printed in abbreviated form like:

{ _id: BinData } --> { _id: BinData }

If, on the other hand, the type of the shard key is say a number, the log messages are useful:

{ _id: 103 } --> { _id: 206 }

This truncation of BinData in the log messages makes it difficult to track sharding problems when BinData is used as a shard key.