Currently, having a broken json file will have no indication of an error other than no ores generating at all. As a QoL feature, sending a message in chat when you have a broken json would be nice.

What version?

2.0.9, Minecraft 1.12.2.

Check your log file / console: There should be messages every time a vein fails to parse, or if a file failed to parse, of even if a vein was specified incorrectly. It should log both a message "Vein X failed to parse", and the root error / exception.
I'm not going to add a chat message as that is not the proper place for error messages such as these.

Alright, makes sense. Checking the log did show the error - I guess I was just too used to crafttweaker logging things in chat, haha.