On top you can watch Jared Polin’s A9 review.
Looking at what the A9 can do that other cameras cannot, then the 20fps is impossible to beat, especially with no blackout between images. It is likely that the DSLR, with its mechanical constraints, can never surpass what might be possible with a mirrorless camera. For various reasons of lens availability and professional backup, the Sony has a little way to go yet, but it delivers some unique possibilities as it is and is a fine camera.
The Sony A9 is a fantastic camera. Is fast, clever autofocus system makes photographing tricky subjects like birds in flight a cinch while its electronic viewfinder allows you to be confident that you’ve got the result you want as you shoot. With a good quality lens like the Sony FE 70-200mm f/4 G OSS mounted, the level of detail in images is superb and thanks to the full-frame sensor, f/4 is wide enough to isolate your subject from its background. Noise is also controlled well and raw files have plenty of latitude for adjustment.