I had far better luck at the very start of the game. The dark lords girls were pretty easy to obtain. Yet, the Ninja only gave me Solveig, and after months I finally got my next drop, which was Housemaid. So I am still missing two from the Gruntt and the Ninja.
I have not obtained a single event girl from drops though, other than the one I was lucky enough to recieve for making sure I hopped on and did the Christmas missions.
It's just crazy, I feared that getting Housemaid during the event basically effed me over.