I don't know how it is for Xfinity, but I work for Spectrum and the low upload is because there is not enough room for it on coaxial cable running at 750MHz or whatever it actually is. A big majority of the bandwidth of the cable, for my company at least, is taken by television and download, which we currently run a docsis 3.0 and a docsis 3.1 segment for download. The upload is a shrimpy part of the band. I know in some areas we are upgrading to support 1.2 or 1.4 GHz, lots more room, so we're able to increase upload in those areas. This is all rolling out now, I imagine other providers using copper will be doing similar eventually to compete with each other. Lets us run more upload plus double the docsis 3.1 segment so we can go into higher speeds for download (like 2Gbps). One consequence of this is we're screwing older TV customers, old cable boxes and also TiVo/cable card shit are gonna stop working.

Not trying to astroturf it advertise it whatever, just sharing what they have been telling us. Upload has always been dogshit because they wanted big download numbers to advertise. I literally get free cable from work but have AT&T fiber installed at my house because I can't handle the instability of the up pipe on coax for some of the shit I do. (Stream to twitch, run a Plex server, etc) it also makes you lag worse in games. Not the overall low speed, just the instability.