Well, in practice, I think the law is mostly harmless with the right interpretation, but if the harmlessness of a law concerning freedom of speech depends on the people enforcing it, it is by definition not a harmless law.
Theoretically, the Bundesamt für Justiz could use a single post that is judged more harshly by a judge than by the moderators as evidence that the moderation system has not been implemented properly. In practice, I assume they’ll be a bit more sensible than that, but (a) how much more sensible, and (b) how much money will FB & co be willing to bet on that?
I don’t give a shit on how much money Facebook is going to bet on that, honestly. I think they will try to do something internally, lobby against the law, and in parallel prepare for a legal battle which most likely they would be willing to fight until the Bundesverfassungsgericht.
I expect them to win, btw. But until then, the process might already be improved. So, basically everybody wins: Maas, Facebook, the SPD, and probably the general (German) public.
That is to say, everybody loses as well. Maybe exactly the general public.