Can you explain your reasoning? Isn't high scalability one of MongoDB's key features?
When you go to the supermarket, you may see products calling themselves "quality" or "luxury" or similar. Words like "powerful" and "scalable" are like that.
Thanks, but you didn't really explain anything. In what specific ways does it fail to scale well?
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.
Scaling has a number of components to it - it's not just about the absolute number of requests. There are always trade-offs (read up about the CAP theorem). My experience with Mongo has been that it has very inconsistent performance. Such inconsistency makes capacity planning very difficult. Also - stuff like failover, sharding etc. are not elegant.
It's not as bad as it's made out to be. It's only if you really are looking to scale out, you should probably be better of picking something else.