As soon as you start having constraints such as "from date must be before to date" and "the quote must have at least one line, and these lines should sum to > 0 and < 1000000" then relational modelling fails. Hard.
Ignorance is often the reason why they find such things "difficult". Many NoSQLers just aren't aware of the existence of the check constraints and triggers offered by basically all relational databases.
Why?