I have created some fairly complex assemblies with Assembly Workbench 2. Per the last comment, with AW2, you can set parts to fixed. You can also set parts to a percent transparent on the view tab.
One hard thing I ran into is when I wanted movable assemblies. I had a suspension arm that I wanted to be able to move up and down while constrained to the mounts. I positioned the arm such that the origin was where I wanted it to pivot, then wrapped it with another object. I would then Use that in the assembly. A problem I have run into is when I make a change in a part that causes the indexing of the lines and such to change. Then the assembly flakes out and I have to reattach that part.
A FreeCAD assembly file is attached here: https://grabcad.com/library/configurable-driver-1