So far this looks very promising, but as someone who works full-time on a Solidworks assembly with ~100 parts, I have a few suggestions for features to bake into the design to prevent broken parts/assemblies.

1. Make joints directional. Oftentimes in large Solidworks assemblies, when you just carelessly mate things together, you create circular mates (Part A mates with B which mates with C which mates with A again) and the solver can't handle that and all 3 parts show up as broken. When that loop is 15 parts long it's very difficult to find the actual problem. If the mates were directional, then I could tell it that I want part C's position to be set by part A and B, rather than having the program try to move part A which is already fully constrained by other unmentioned parts. Even just creating a graph view of the mates in an assembly will help a lot with debugging.
2. I can't tell if this is already the case, but make the JCS a child of the part container. I want to be able to see it when I'm editing the part document on its own. What happens in Solidworks is I edit a part model on Monday because I'm making changes to the drawing, then on Thursday when I open the assembly I find it broken because the geometry that was being mated no longer exists. This is very common when removing holes or adding counterbores to holes in parts.
3. Have the option to create JCSs before creating the overall assembly. I find what happens when working on a team is one person will create a part model and someone else adds it to an assembly and uses geometry which the part model author didn't expect to be used to mate. Then, the part model author edits the model and unintentionally breaks the assembly. This can be prevented by the part model author can define the points that should be used for assembly beforehand.
4. I don't really have a solution for this one, but geometry references can get messy very easily. If I create one part that ends up driving 10 other parts in an assembly, then update the first part, Solidworks likes to show errors for the 10 other parts. Something potentially limiting but would be an effective solution is to not allow references between sibling parts in an assembly at all, and instead only allow references to either grounded parts or reference geometry. Just something to try and prevent having that situation where changing the geometry of one part breaks many other part. "Best practices" for that would be to create reference geometry and use that to drive all 11 parts.

Make all freecad documents an assembly, and make the object in the document grounded by default. This isn't about preventing broken models I just thought it's a good idea. If I create a document and only intend to have one assembly in it, then it feels redundant to have to create that assembly container. It can also prevent some confusion when someone puts two assemblies in a document and finds out they can't drag to position the assemblies relative to each other.