One thing is that congestion control choice is sort of cursed in that it assumes your box/side is being switched but the majority of the rest of the internet continues with legacy limitations (aside from DCTCP, which is designed for intra-datacenter usage), which is an essential part of the question given that resultant/emergent network behavior changes drastically depending on whether or not all sides are using the same algorithm. (Cubic is technically another sort-of-exception, at least since it became the default Linux CC algorithm, but even then you’re still dealing with all sorts of middleware with legacy and/or pathological stateful behavior you can’t control.)
I mean, if you're trying to be a good netizen, you try to tune your congestion control so it's fair enough in at least a few scenarios. You want it to be fair relative to status quo streams when status quo is dominant or when your new system is dominant, and also fair relative to new streams in the same conditions. This is a challenge of course, and if something in the middle is doing its own congestion control, that's indeed its own layer of fun and pathology.