Already shot a local Football (Soccer) match with it and I kinda agree with you BUT the issue is BECAUSE of the faster frames rates, shutter speeds and HIGHER ISO people TEND to use with this camera! At 20 fps for Soccer, I'm shooting 1/200th of a second minimum (I STILL like SOME motion blur in my photos for that SOCCER ACTION look!) AND because it's mostly cloudy/rainy in Metro Vancouver, I'm also shooting ISO 3200 or even ISO 6400, so I am DEFINITELY noticing more noise in the shadows and dark colours of the player uniforms on my photos versus the 1D Mk2 ...BUT... that's because I'm boosting my ISO setting all the time AND using a faster shutter speed in my high-speed sports/action shots BECAUSE the Canon 1Dx mk3 ACTUALLY IS capable of handling those sort of bad-lighting/late cloudy afternoon of super-intense sports/action scenarios that the 1Dx Mk2 WAS NOT ABLE TO DO !!!!
For Editorial Purposes, I don't actually find this a problem since image distribution is mostly online via end-user 1920 by 1080 pixel displays or in PDF files, so we DOWNSAMPLE the image using a Lanczos-3 Algorithm down to EXACTLY 50% on the horizontal and vertical axes to 2736 by 1824 pixels which averages out pixels (i.e. a form of fast and cheap noise reduction!) and use an UNSHARP MASK to get my edges and details back!
Works so far!
If I shoot 20 fps burst at ISO 1600 at 1/800th of a second on semi-bright days or indoors (Basketball), i'm getting action shots with the 1Dx3 I never could on a 1Dx2 so it's a trade-off. (i.e. up the ISO the faster your shutter speed up to ISO 6400 at 1/8000th of a second on BRIGHT days for sports like F1 and Skiing or hockey !)
I just do more post-production before sending my keepers into the editors!
For birding (fast moving ones such as diving falcons or hummingbirds) the 20 fps IS DEFINITELY A GREAT NOTCH in Canon's belt BUT you have to up your ISO and your shutter speed which means you also get more noise!
I should ALSO NOTE it's actually the HEIF image file format that mostly sucks so I shoot FULL RAW now! HEIF kills the shadows and highlights too much because of it's "enhanced" DCT (Discrete Cosine Transform) tables AND has "noisy macro-blocking" which I think is what your real issue is! SHOOT RAW PHOTOS to get less noise !!!!
Since HEIF tries to stuff the same visual quality image into HALF the space what is usually used by the older JPEG file format, of course there are sacrifices in image quality being made! Canon is trying to give you more photos per battery charge and more photos per CFexpress card when shooting on HEIF image compression, Ergo, you NEED to SHOOT RAW to get much higher-end image quality!
Canon will LIKELY supply a camera BIOS update later this summer to fix SOME of the HEIF file format issues and SOME of the inherent-to-HEIF file format "Noisy Macroblocking" problems ....BUT.... that won't happen until AFTER the Olympics around September 1st, 2020 my bet! Again .... SHOOT RAW --- you WILL get better looking, less noisy photos!!!