Profitable companies have usually figured out that it's better to buy a solution that solves their problem rather than: (a) pay for engineer time to integrate a solution that isn't quite right, (b) take the opportunity cost hit while waiting for (a) to be done. These costs can pretty quickly dwarf the purchase costs of licenses and hardware. Of course if there's a free solution that just slots right in, double bonus, but for the most part the cost of hardware/software is noise next to the cost of people to maintain it.
So relational data is hard but implementing appropriate consistency checks in your application is easy? Or do you just skip that second part and hope for the best?
Spending millions on engineer time to avoid buying a real database is not good finance either. At least with this approach you can start with the (free) MySQL and only pay once you're sure your idea has traction.