The viewfinder EVF has a mode called 'live view' when shooting at 6fps or 8fps...it's very slightly different in feel from an OVF but similar - rather than having the shutter blacking out the view while it trips up and down, the EVF displays a current or live view in between blackout frames - so the effect is for me quite close to shooting with an OVF. This is different than what's typically called 'slideshow' effect, whereby the EVF displays the last photo taken in succession, so you get the EVF a little bit behind the action. This was for me a very big improvement when it debuted on the A6300 compared to the previous bodies, which were slideshow. Much easier to stay with an erratic, small bird moving all about while firing bursts. If I switch to the 11fps mode, it comes off looking almost like continuous video stream in the finder, but it is in fact doing a fast replay of each previous frame at 11fps, so though it looks smoother, it's always a bit behind the action in the slideshow style and harder to stay with a fast or erratic bird...so I tend to stick to the 8fps mode with the live view EVF mode.
As this year was a bit limited for opportunities, with almost all parks and wetlands closed for at least some big part of the year with Covid restrictions, I didn't get as many opportunities yet with the A6600 as I only just got it in mid-December 2019...so far, it's done very well for birds against busy backgrounds with lots of branches, leaves, etc - I've gotten a few opportunities with smaller birds like kingfishers, grackles, doves, and one or two swallows with busy backgrounds and the tracking does very well at sticking with them, and no issues following them with the EVF while firing. I had 3+ years with the A6300 before it, and it also did very well even with non-sky backdrops - it didn't have a usable 'tracking' mode like the A6600, but continuous focus was pretty good at sticking to the closest subject and not getting distracted easily, and it had the same 8fps live-view EVF mode...I had shot purple martins, barn and tree swallows, etc in flight and found the system to work very well for me, where previous mirrorless cameras with the older slideshow-type EVFs were definitely more challenging trying to keep the bird in the frame.