[Klinky]

A few of the major issues I've seen around are the global write lock which is starting to get taken care of. The reliance on the OS to cache instead of intelligently caching based off data. Poor fail over support. Sharding not being a "turn key" solution like it was supposedly advertised as, misconfiguring it can lead to poor performance. Bringing extra nodes online can take a long time for data to migrate over to the new nodes.