> If the domain needs transactions [...]

Considering we speak about daemon software (services exposing some API to readers and writers) that provides CRUD behavior to the user (end-user or other developer), isn't that nearly always the case to guarantee write access to concurrent writers without the risk of crippling your data?

Furthermore I am not sure how this relates to FP.

> It's much easier and safer to declare your constraints rather than trying to enforce them.

But that is a strong point of RDBMs implementing SQL. You have some kind of schema (think type) and use selected functions (select, update, delete, create, etc.) to transform the data.