Well, in a commercial setting, several concerns apply. First, it may be illegal for you to actually have the code you work on leave your enterprise. This can be soft (breach of contract) or hard (criminal act), or in between. Second, that backend may give your code to others. That may cause a lot of issues. If you did this voluntarily, trade-secret protection may be gone. If there were vulnerabilities in there, attackers may get access to and then craft exploits. I am sure other problems may exist. And third, what if you get code that somebody else had leaking this way? Suddently, you may be using code under copyright by somebody else. Or take, for example, that you bublish the code, attackers find a vulnerability and the original source gets attacked and finds out what you did. You may be legally in the clear, but years and years of litigation could result before that is established.
So, yes, the model is a massive problem. But so is sending your code to it or getting code from it.