Earlier quoted context omitted.
It appears to be a pretty serious issue. The SQL injection alone is bad but the ability to run basically any PHP code through callbacks makes the problem that much worse.
SQL injection alone is often enough to get you RCE if your MySQL account has FILE permissions enabled (often true). Something like `SELECT "<?php eval($_GET['x'])" INTO OUTFILE /srv/www/backdoor.php`.
I don't think it's that often. By default, a new MySQL user will not have FILE privileges granted.
Most of the time you see this due to the developer being lazy and just using the "root" MySQL user.