Read performance compared to a lot of other offerings is a significant issue.
However, one of the biggest issue is the entire sharding design that is incredibly delicate and wouldn't pass even the most basic high availability requirements. I could go into a lot of detail about why it's bad but it would take too long.
After that, it's the hardcoded limitations that prevent true multi-datacentre sharding, the previously mentioned CPU issues with mongos, broken replica selection, inability to control primary/replica setting manually, etc.
However, one of the biggest issue is the entire sharding design that is incredibly delicate and wouldn't pass even the most basic high availability requirements. I could go into a lot of detail about why it's bad but it would take too long.
After that, it's the hardcoded limitations that prevent true multi-datacentre sharding, the previously mentioned CPU issues with mongos, broken replica selection, inability to control primary/replica setting manually, etc.
The list is virtually endless.