I want to preface this by expressing my extreme gratitude to all the developers who contribute to making FreeCAD such an amazing application.
I understand that there are very few active developers, so I hope this doesn't come across as ungrateful in any way: I understand the value of trying competing implementations, but I think there's been enough experimentation at this point. The developers of the separate implementations should team up, select the best features from each, and create the official Assembly workbench.
Over time, I have had to either completely redo my designs or retain old versions of FreeCAD in order to be able to open old files. I've got designs based on Assembly2, A2plus, and one incomplete model made with Assembly3. I found out about Assembly4 shortly after I started the one that uses Assembly3 and discovering that there was yet another Assembly implementation made me question whether it was even a good idea to start on that design, since it's unlikely I'll be able to open the file without jumping through hoops in a year or two.
I really enjoy using FreeCAD and would like to use it for all of my designs. I'm only able to use it for designing simple parts, though, since I feel like it's just a matter of time before files made with any assembly workbench that's currently available will be useless. It's essentially a waste of time to learn any of the assembly methods that are currently available in FreeCAD.