Why PS5 & Xbox Series X's HDMI 2.1 Could Cause Problems
Both new consoles support 4K resolution at 120 FPS, 8K resolution at 60 FPS, and variable refresh rate ("VRR," which can help prevent issues like screen-tearing and input lag), but all of these features require both a compatible TV and video output cable (HDMI 2.1). The increased gigabit-per-second rate of HDMI 2.1 over HDMI 2.0b allows it to deliver 4K120 and 8K60 from the console to the TV, but that doesn't mean anyone who plugs an HDMI 2.1 cable into their TV will experience these benefits.
Only select Tvs & Monitors support HDMI 2.1 (which happens to look nearly identical to HDMI 2.0b). Not all TVs with HDMI 2.1 ports actually support features like VRR and auto low-latency mode. Making matters worse, even if a certain TV says it supports VRR, that could mean it supports the full 120 FPS next-gen consoles allow, or it supports only up to 60 FPS. It's easy to imagine a scenario where an unaware gamer walks into an electronics store and remembers Sony telling them the PS5 can output 4K120 with VRR, but then ends up coming home with a TV that can't actually take full advantage of that.