I have broken one reed, Melodica-Me. I’m sure it was failed or defective, because it suddenly went seriously flat and then the same happened again after I tuned it up. I broke the reed by lifting on the end trying to widen the gap. I think the greatest danger is from filing crosswise or from catching the reed tip while filing at the open end; the scraping or filing motion at the open end should always be in line with the reed and toward the end, and it’s important not to pull the file back carelessly at that end.
I agree with you that the scraping or filing is the easy part; getting clear readings with a meter or by ear before tuning and after scraping is the hard part, obviously easier the better your ear is. Some of the tuning apps have an ear training function.