> Is this just a "distributed computing is hard" situation?

I think this is "gaming performance benchmarks and being right is hard".

MongoDB has spent a lot of effort on making sure it looks good in benchmarks, and that includes using worse defaults than any other distributed data-store I know of.

As long as MongoDB continues to wish to "win" at performance benchmarks by so much, they'll have trouble being able to provide a correct distributed system by default.