Correction, you can get 4K@120hz with HDMI 2.0, but you won't get full chroma 4:4:4, instead 4:2:0 will be forced.

In my case I have an htpc running linux and a radeon 6600 connected via hdmi to a 4k @ 120hz capable tv, and honestly, at the sitting distance/tv size and using 2x dpi scaling you just can't tell any chroma sub-sampling is happening. It is of course a ginormous problem when on a desktop setting and even worse if you try using 1x dpi scaling.

What you will lose however is the newer forms of VRR, and it may be unstable with lots of dropouts.