This area is called Lobau. It’s a left-over from before the regulation of river Danube. It’s also an important national park, that has been endangered multiple times by power station and highway construction projects.
At the moment, the project in discussion is a tunnel below the river and the park. From my point of view, the main objection against it is that it will create even more traffic. On the other hand, if you regularly need to use the always congested current tangential highway, then even the distribution of traffic sounds like a good idea.
This leaves air pollution from the tunnel’s vents, but progressing electrification of traffic will help a lot.
You see, as a Green voter and as being directly affected by current congestion, I’m divided on this project but tendentially supportive.