Trust me. I have a house there. 2 or 3 years ago the last snow was on about January 20th (I remember as was just before my birthday) and they managed to drag the season out right to the bitter end.
Niederau slopes are on the shaded side so frankly it's just not an issue. They have easily enough depth to last the season even if it never snows again.
The only real danger is warm weather and rain - THAT is what truly buggers things up. As long as it stays cold then you are sorted.
Personally I hate normal bashed pistes, as a boarder I live for powder, but the slopes are always at least usable in Niederau. Some people like skiing on pistes but it just bores me rigid without new snow.