As with any tech, you can't trust marketing if you plan on pushing it to the limit. You need to either test it yourself, or in some rare cases there are reviewers who have already tested it. Most "reviews" for tech are extremely superficial though and certainly won't be testing HDMI inner workings.

For HDMI 2.1, there are a bunch of monitors being sold under that banner that don't have the full 48 Gbps bandwidth. For example the Gigabyte M28U is limited to half of that at 24 Gbps. [1] Gigabyte certainly doesn't want you to know this. On their specificaion page they just list it as HDMI 2.1. [2]

Similar nonsense was going on during the transition from HDMI 1.4 to 2.0. I really wanted HDMI 2.0 output on my laptop and held off on a purchase until that was possible. I finally bought an ASUS Zephyrus GX501 laptop. It has a Nvidia GTX 1080, which does support HDMI 2.0 output. The marketing for this laptop also seems to suggest that they're utillizing this potential, with claims like "You can also run a large ROG gaming monitor with NVIDIA G-SYNC™ via DisplayPort™ over Type-C™ USB or use HDMI 2.0 to connect 4K TVs at 60Hz." [3] The specification page mentions the HDMI 2.0 port. [4] However in reality I found out that this HDMI 2.0 port is limited to HDMI 1.4 bandwidth. It supports HDMI 2.0 features like HDR, but not the bandwidth. 4K @ 60Hz is possible only with 4:2:0 chroma subsampling and you're limited to 8 bits, so no HDR.

I'm not the only one who found this out either. There are plenty of others on the ASUS forums. [5] Hard to say whether this was intentional misleading by ASUS marketing, or whether engineering messed up, or whether the feature ended up being cut due to time constraints. In any case, they still haven't adjusted the old marketing pages for this laptop that never shipped with HDMI 2.0 bandwidth.

Reviewers don't tend to check things like this either. For example The Verge reviwed this laptop [6] and wrote: "Asus has a nice array of ports on the Zephyrus — four traditional USB 3.0 ports, a Thunderbolt 3 USB Type-C port, HDMI, and a headphone jack." They're just regurgitating the marketing material. There's no depth to it, the claims aren't verified. So people end up buying the product and then get confused why it isn't working.

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[1] https://www.rtings.com/monitor/discussions/q-D1CBeE2EiGMYgn/...

[2] https://www.gigabyte.com/Monitor/M28U/sp#sp

[3] https://rog.asus.com/laptops/rog-zephyrus/rog-zephyrus-gx501...

[4] https://rog.asus.com/laptops/rog-zephyrus/rog-zephyrus-gx501...

[5] https://rog.asus.com/forum/showthread.php?96916-GX501-Zephyr...

[6] https://www.theverge.com/2017/8/25/16201656/asus-rog-zephyru...