[norkakn]

2 is wrong. Mongos shits all over itself after only a few nodes and moderate traffic. It's balancing is incredibly broken and will kill a small cluster very quickly.

Source: tried to use it at scale

[threeseed]

The balancing is broken if you use an inappropriate sharding key. Which is no different to every other system.

And I've used it under very high load with zero problems other than data model changes.

[norkakn]

I think we had one of the larger mongo installs around, so you just might not have been at the scale to see these issues.