I'm not sure this is an issue per se but might just be a poor understanding of building fronts and building connections with regard to custom structures. I've attached both a crash log and a custom userassets.json file. I get a crash related to building connections with the garage building when I put it into the citystyle as a potential building to put into cities. I get a similar crash with the reactor building.

A few things I can think of:

1. There are some requirements for custom structures not directly specified which requires blocks or air in specific locations to allow for doorway connections, similarly for building fronts, and my custom buildings are not following that requirement in some way.
2. Is there a way to specify that certain buildings cannot have adjacent buildings?
3. Similarly is there a way to make it so certain buildings do not have any type of front applied to them or is there a way to make the front building specific using conditions?

Crashlog https://pastebin.com/yPkzdfZx
Userassets.json https://pastebin.com/qGpxhW8s