I've worked on a service like this. 800k lines of PL/SQL and Java stored procesures (running inside the database, so you could call Java from PL/SQL and vice versa), powered by triggers.

* Testing is god-awful. To test a simple thing you had to know how the whole application worked, because there's validation in triggers, which triggers other triggers, which require things to be in a certain state. This made refactoring really hard/risky so it rarely got done.

* There's a performance ceiling, and when you hit that, you're done. We did hit a ceiling, did months of performance tuning, then upgraded to the biggest available box at the time, 96 cores, 2TB ram, which helped, but next time the upgrade won't be big enough. You're limited in what one box can do (and due to stored procedures being tied to the transaction there's limits to what you can do concurrently as well)