It does sit on the plastic rack but it is not an issue at all. There is a steel frame under it and it is very tough plastic. And the load is spread out. I don't think you could hurt it if you filled it with lead. ???
I have had mine on from the start. 62,700 miles so far with no issues. And I have carried a lot of heavy stuff in it. Like when I make a run to the pool store, I take two chlorine containers and both fit nicely inside the Givi. When full, that is about 35#. I think that is about as much weight as you can physically get into it.
I have ridden coast to coast and back 3 times with it and it always carries my computer back pack with all my associated stuff in it. Call it 25#. Zero problems. ::012::
The only down side to the large Givi that I can see is the look, and that is personal taste. It doesn't look as utilitarian or as "Adventurie" as some of the others but I don't much care about that and it sure works great for me.