Xfinity Stream is working for me using Safari. I'm running MacOS 14.4 but haven't had any issues before except for it freezing every now and then but it's rare.
Do you have Pi-Hole or something similar installed on your network? Any VPN or Private Relay turned on? Ad Blockers? Try turning those off and see if it fixes it.
I do have AdGuard and StopTheMadness extensions installed but I disabled per-site content blockers and allow all auto-play for the Xfinity site. I don't know why I did this but these are my current settings.
Also, if you ever played around with the Feature Flags settings in Safari, try resetting them to default.