The big problem in San Francisco is that during severe rainstorms the flow rate overwhelms the treatment plant so they have to dump the overflow. but at that point it is mostly rainwater so it's not quite as bad as it sounds.
For example, how many cubic metres of rain water makes adding one human turd OK ? A million seems fine, doesn't it? Like, who cares about just one turd in so much rainwater. How about a thousand, that's a swimming pool (not Olympic, but decent sized) with a turd in it, is that OK? Start to feel a bit uncomfortable with how much shit there is in the water? What if it's a hundred? Are you sure San Francisco, or London, can promise you their CSOs have at least that ratio?
(Disclaimer: I vaguely worked on software projects that were CSO mitigation adjacent. Which at least explains some the interest in the projects.)