I worked in different banks for the better part of a decade, lots of internal applications, each with their own DB. They all had PK/FK/unique/check constraints

I worked in different banks for the better part of a decade, lots of internal applications, each with their own DB. They all had PK/FK/unique/check constraints, and they were still a clusterfuck to understand and maintain. Lots of outright stupid and dangerous code to work around the issues. Still, if there were no constraints, it would have been even worse, as some 'intelligent' people temporarily disabled constraints to update data, resulting in data integrity hell.