An Uninteresting Problem
The key words here are "well enough." Not everything fits neatly into the dichotomy of "good enough" and "not quite good enough." A library or technology may do several things well, but miss one very important thing.
SQL databases are often cited as an example of this symptom. They perform very well when used as prescribed; they're durable and provide all kinds of mature data-manipulation capabilities, such as multi-table transactions, multi-part indexes, and numerous other useful features. However, they don't handle very large amounts of data well. The database is often capped by the power of the single machine on which it sits. Consequently, distributed datastores started popping up all over the place—Cassandra, HBase, and Voldemort among them.