I've used Firebird SQL with Synapse since March 2006. I've had absolutely no database problems at all. This includes many crashes of Synapse while playing with alpha releases etc. Firebird is easy to maintain and darn near close to zero administration. I do hourly backups, but have never needed to use them.
And I guess the good (or bad, for me) news is that I violate all of Graham's suggestions.
(1) I use a Windows XP box to host the Firebird Database.
(2) The computer was cheap, not overly high-powered.
(3) I often (mostly) use Synapse right from the "server" computer !
(4) I often run tonnes of other apps on the "server" computer as well.
Admittedly I am going to migrate to an apparently more robust system, more along the lines of Graham's suggestions.
This is just a real life testimony that despite not following the most robust guidelines, Firebird has been rock solid.