Oracle historically has poor support. Here's an old example with an ADO bug, they never fixed it even though the support ticket was open for almost 2 years!
http://randomtechnicalstuff.blogspot.com.au/2009/11/nasty-or...
http://randomtechnicalstuff.blogspot.com.au/2009/11/nasty-or...
With postgres and mysql you can find many companies competing to offer great service, and who can contribute their fixes and improvements directly should they so choose. The stronger the output of these communities and the organizations that support them, the more pressure for even the companies like Oracle to improve ( or perish :))