When discussing this topic with our operations team with cPanel, the overwhelming consensus was 'don't implement hard limits with database' or screw with things like setting databases to read only.
It has a very real possibility of corrupting databases, causing applications to fail in strange ways (unable to insert) leading to data loss, lost orders, backups failing.
Most databases are only a few hundred to a few gb. Put some good monitoring (disk space) on the server and run a report to look for huge databases once a month. If you find one, either suspend the account or send them a warning. The real problem to look for is huge error logs (which you can script to delete) and large mailboxes.
Inode limits are a far bigger issue at scale IMO
