Agreed. Seeing a nice, clean user-facing API tells you practically nothing about the performance characteristics and failure modes you can expect to see.

The best database engine I've encountered in this respect is SQLite. It has plenty of information about what design tradeoffs it makes, and why, e.g.:

http://sqlite.org/lockingv3.html

http://sqlite.org/fileformat2.html

http://sqlite.org/atomiccommit.html