I have a system that contains two databases, MySQL and Firebird. MySQL table reaches about 50 Million record and it works fine, but Firebird becomes very slow and hang the system up when the table reaches hundreds of thousands of records, also I noticed that it consumes more CPU than MySQL.
In one case I found that Firebird did not use a secondary index to speed up the query. In a query that needs to return 20 records it takes about minute to execute because it contains descending order, when I turn it to ascending it takes seconds, I have created descending index, but I get the same delay.
The most important feature is the on-line master/slave redundancy which exist natively in MySQL. We are relying on slave database to generate reports and to make full backup, to keep master database busy with important tasks.
Actually I'm using MariaDB but it is the same like MySQL.
Another factor is that MariaDB/MySQL is used by large web sites such as wikipedia, and Twitter. Customers tend to ask us what database you are using, second question is what language. Third (optional) question is what operating system in the server side.