If you are running into a problem that is bringing your site down and it's truely a problem/bug with a commercial database then you are in trouble and unlikely to see a quick fix as these products are complicated and while the problem might be existential to you, it's not to the bigger vendor.
However, it's much more likely it's a configuration issue - ie you didn't understand the product properly and have configured it incorrectly - and here support can really help.
The point I'm making above is not that you should be writing fully fledged all singing and dancing databases - but that you might be able to get away with something very simple, more reliable and faster in your specific use case.
If on the other hand your use case is complex and a good fit for something pre-existing - then sure you shouldn't be writing it yourself.
Don't forget, do you need someone you can call for support (or throw under the bus) when problems come up?