Baccano! is simple and done well. The first season of Durarara is simple and done well as well though from there too many characters and plots are introduced to the point where it's hard to keep up with it and I didn't care about what happened to all the new characters. DBS is probably a complex mess too. It's much easier to make a mess when there are too many characters.
Baccano is simple, but it's not done too well. Of the three different arcs, one is awful. That's a third of the show that isn't done well and it ruins the mood for the other two arcs just by being next to them. Fortunately, the other two arcs are good.
The first season of Durarara was neither simple nor done well. While Baccano had only three different arcs running at the same time, Durarara tried to follow many different characters at the same time and the chronology was extremely messy. Baccano's arcs ran chronologically, but parallel to each other. Durarara's plotlines go in zig-zags and it's extremely messy.