@matt39291... fantastic! Glad to hear Fusion is working well for you. Our team also primarily leans on Fusion for most of our needs, and we list many of the same advantages you do.
HOWEVER 🙂
I also use Inventor Professional for areas that Fusion is weaker. One of those, specifically, is large-assembly management. Maybe you haven't run into the ceiling yet, and depending on the nature of your assemblies, you may not. In my experience, there are three specific pieces of functionality that Inventor (or SWx, as Peter suggested) will offer that Fusion currently doesn't.
- iMates: as Peter suggested.... these make a world of difference, especially when you can create compound/named iMates that allow placement at a whole bunch of holes easily. No equivalent in Fusion.
- Content Center : While Fusion does offer the ability to link external parts, it doesn't have the tools to handle 1000 fasteners. As you've noted, simply placing them is.... doable, but not ideal. Imagine getting a few hundred placed, and then deciding to swap half of them out with another size 🙂
- Substitute/proxy parts : Inventor offers 'Shrinkwrap substitutes' and other tools that allow creation of lightweight placeholder parts when the component count starts to get high. Actually modeling the #10-32 machine screw threads is great for zooming in a doing detail design. Once you've got the fastener specified, though, having 1000 of them in your assembly is just a lot of geometry that really isn't adding much value, just slowing you down. Being able to quickly swap in a 'lightweight' version of fasteners can improve performance. A LOT.
- Levels of Detail (LOD) : another pretty common tool for optimizing performance is to create LOD's.... i.e. just freeze/turn off all the fasteners when you're creating an overall plan or elevation view. You can definitely use selection sets to create groups that can easily be switched on and off, but I'm not sure if there's a way in Fusion to save that as a 'defined' LOD. (meaning, I haven't found it, but I also haven't looked very hard for it)
For what it's worth, there are several folks out there that are interested in the upper limits of Fusion's assembly capabilities. Martin Molin (Wintergatan) is a truly talented guy who's right in there pushing it. He's done some amazing work in Fusion on his Marble Machine X, and explored this a bit with @jeff_strater in THIS video.
For my part, we will continue to use Fusion for all the things it's great at, and flip over to Inventor when needed.
Todd
Product Design Collection (Inventor Pro, 3DSMax, HSMWorks)
Fusion 360 / Fusion Team