Which db engines, and why? None of the newer ones have had enough time to prove themselves yet, hence why you don't see mass adoption.
Unless you are going over 100k-150k hits a day (and that is being really conservative), SQLite is better by quite a bit in my opinion. Most people using MySQL are just being dumb and fucking silly, like they are using dynamite when all they need is a little hammer.
It is simple, fast and proven, but passed over by people who would be better off using it because they are blinded by hope, thinking they are going to be the next google or facebook or some silly shit...
Plus it is 'murican.
It was originally developed for missile defence systems or something like that, hard to get much more mission critical than stopping an ICBM.
MySQL is swedish communist crap.
If you actually do need a big, heavy duty database engine, postgre is just plain better.
MySQL is like a v6 mustang or a porsche boxter or something. People who can actually drive a real sportscar don't buy them because they suck, people who should be buying a fucking camry do buy them because they like to feel special.