My take on the opening question is basically that it has got steadily more complicated to mod graphics in Civ IV > V > VI.
Civ 4: You could do reskins, re-rigging and even animation edits using
Nifskope without knowledge of any 3D modelling tool. This meant that the barrier to creating original 3D art assets was very low hence the wealth of models and reskins for that game and the thriving community of that time.
Civ 5: The Granny Engine format meant that you need to edit the .gr2 file just to do a reskin and it wasn't possible to edit the texture and see the results immediately in the same tool. You could see the results in Nexus 3D Viewer but only if you used the older one before Firaxis broke it. For any edits to models you had to use Blender and Nexus Buddy 2. On the upside the Art Defines were fairly simple and could be modified by SQL.
Civ 6: Still Granny Engine so the modelling process is familiar to anyone who was comfortable modding Civ 5 3D assets, but now rather than the Granny .fgx files being used directly in the mod they have to be cooked into .blp libraries. This process is quite good and really flexible once you "get it" but "getting it" seems to be hard for many. To get a re-skin in game I will now need a Modbuddy project with a bunch of files Units.artdef (including Unit, UnitMember, UnitAttachmentBin), XLP, Asset, Material, Texture, Mod.Art.xml. Art Defines are now more complex and can't be edited via SQL. Although the Asset Editor exists I'm not sure how easy it is to use (personally I only use it for Texture Import as I find it quicker to edit the asset files and Art Defines directly). On the upside the
Asset Previewer is very useful although it doesn't allow you test everything out the game e.g. Natural Wonders.
Having worked with Civ 6 for a while I find it much better for modding 3D graphics than Civ 6 because we have better or equivalent tools together with the possibility of visualising stuff outside of the game. Also the depth of moddability is much greater overall. We can do much more with Terrain in Civ 6 than is possible with Civ 5 for example.