HBase guy chiming in. Sharding in HBase is a bit different than the other NoSQL guys. HBase is super fast when doing random lookups and scans, so the developer is in complete control of his failure or success, depending on the row key design.
Good thing about HBase is that it's designed to hold a massive amount of data, I've seen people with hundreds of thousands of columns on a single row, it's pretty remarkable.
I suppose you would have an issue with MongoDB specifically if you were trying to rebalance from day one. But generally you wouldn't. I know Foursquare is using MongoDB's Auto Sharding in production and they would definitely have a lot of writes.
And the number of tables DOES matter for sharding if you are implementing it yourself.