Green roofing of flat roofs: Handling external drains

We have two houses with green pitched roofs in the nearby neighborhood. Looks great. Slope... hmm less than 45° definitely.

Of course, no question, it works. The question was whether the effort is higher: it is, and a green roof offers more risks of leakage. Everything else is a matter of personal opinion. Green roof = environmentally friendly, expensive, riskier! With pitched roofs - that's why I formulated it so cautiously earlier - the effort is significantly more expensive because the roof structure is completely different, more complex.