In the past people, myself included, pretty much ruled Sony out for difficult moving stuff. Wildlife, aviation and sports. Poor AF and lack of reach with native lenses. (Although arguably with the a7r2 you have more reach with shorter focal lengths than certain dslrs with mo get ones)
They were always considered landscape weapons and a lot of people had a 2 brand set up. Dslr for moving, Sony a7 for still - low weight and stunning iq.
Now, the a7rii started making inroads AF wise and wedding photographers and portrait photographers were using them to great effect. The hints were that a mirror less system can actually provide more accurate af.
With the a9 it appears so Sony have made an action camera to better 1dxii and d5. Now we have great af, fast frame rate but still a lack of reach. Expect sports photographers who work within 200mm to love this camera.
@johnf3f it will be the niche you are in, wildlife/birds along with other specific long lens work that will still rightly stick with dslr. I'd suggest that if Sony sigma or zeiss produce some decent Long lenses that a Sony alpha would be good for you, the lack of weight over a dslr is great and if the lens isn't mounted to the tripod then there is no strain on the camera lens mount.
I miss my canons at times but for every frustration my a7ii's cause me all is forgiven when I look at the images at 100%.