It's reasonable to not have auto increment id's, but it's not clear to me if there is benefits to have 2 IDs, one internal and one external. This increases the number of columns / indexes, makes you always do a lookup first, and I can't see a security scenario where I would change the internal key without changing the external key. Am I missing something?
You always have the information at hand anyway when doing anything per project. It’s also more user friendly to have every project’s issues start with 1 instead of starting with two trillion, seven hundred billion, three hundred and five million, sevenhundred and seventeen thousand three hundred twentyfive.