When should you NOT use RavenDB?
That is a much more interesting question, especially when it is me that gets asked. Again, I am might be biased, but I really think that RavenDB is suitable for a whole range of problems.
One case that we don’t recommend using RavenDB for is reporting, though. In many cases, reporting databases need to answer queries that would be awkward or slow on a RavenDB system. For reporting, we recommend just throwing that data into a reporting database (either star schema or a cube) and do the reporting directly from there.
Most reporting tools will give you great support when you do that, and that is one sour spot (the reverse of a sweet spot) for RavenDB. We can do reporting, but you have to know what you want to report on beforehand. In most reporting systems, you give the users great deal of freedom with regards to how to look, aggregate and process the data, and that isn’t something that you can easily do with RavenDB.
So far, that is the one place where we have said, “nope, that isn’t us, go look at that other thing that does this feature really well”. RavenDB is focused on being a good database for OLTP, not for reporting.