If your customers are behind CGNAT, and you have a good number of them, it would be pretty easy to run into collisions. CGNAT doesn't have any problem using the same source ip:port for connections to different destination ip:port, because there's no reason not to.

I agree though, that most OS stacks are very conservative about using the same source ip:port for connections to anything else. It works ok until you need to make lots of connections, then you have to manage ports in userspace.