LOADS of data - especially for industry-verticals that have heavy integration with credit-card companies (e.g. airlines include ticketing and even seat info in the metadata they submit to banks: this dates back to how air-miles rewards cards originally worked in the late 1980s) and there was tight integration between banks and airlines - even (or so I'm told) to the point of where banks' in-house "main" databases and ssytems have dozens of very, very hardcoded fields because of that early collab work - which then ossified in-place and now everyone's too scared to remove those now unused fields for fear of breaking everything.

...which leads me to believe that Cobol does not lend itself well to unit and integration testing in isolation then.