With over twenty years of professional experience administering and developing against databases as well as working on their internals, eXtremeDB is the only system I've found that gives developers the ability to build almost anything with very few constraints. Likewise, McObject's support is exceptional. You can ask as detailed of a question as you can imagine and get a solid answer, in many cases, from the engineers themselves. Additionally, McObject offers a source code license, which gives companies peace of mind and enables them to understand what's going on under the hood, allowing them to perform compile-time optimizations for specific scenarios.

eXtremeDB is the most flexible database I've ever worked with: it can be embedded multi-threaded in-process, out of process, client/server, clustered, etc.; it supports SQL, vectorized operations, multiple transaction managers, multiple data/index types, etc.; it's well-documented and has samples for each feature; it's available with full source code.

Honestly, I've never had an issue with eXtremeDB in terms of features or functionality.