Well, I've made my transition now.
5D4 with 11 Canon lenses to A73 with 9 Sony lenses + 1 Canon lens.
I found the MC-11 works very well on most Canon lenses adapted, I had them side by side for a year and shot weddings with them at the same time. Albeit on different levels, the later on in the cross over, the less I used the Canon as I was slowly accumulating more Sony lenses and thus adapting less.
The joy of having both system in the car, together they are worth 3 x the value of my car lol.
Sigma 20/1.4 Art - This isn't a lens I used much even when I had it on Canon, I use it for rooms when I need to get back further but want the extra 2 stop over the f/2.8 in the 16-35
Canon 24/1.4 L mk2 - It's fine in the day but in low light it struggles to hit focus. One time shooting in twilight and the A73 was just not hitting focus.
35/1.4 L mk2 - This was fine, i could have kept this but it's quite a long lens for a 35mm and on the adaptor it becomes almost telephoto length.
Sigma 50/1.5 Art - This was very good. I guess it should as it is a Sigma lens
85/1.2 L mk2 - It works fine, sharper than i could get on the Canon because it was hitting focus with eye-AF with that 1.2 bokeh. It does however shows the lens' weakness as it is an old design (like 20 years) and it's not as sharp as the 85GM
100/2.8 L Macro - I only use this lens for wedding rings shot but I struggle with the MC-11 with this to focus even on average items around, it hunts around too much. It's not a problem if i had stick with it using as manual which was what I use it for.
135/2.8 L - Didn't use it on the body in anger so can't comment
16-35/2.8 L - Fine
24-70/2.8 L - Fine also
45/2.8 TSE - manual lens.
The only thing that stood out for me is how I struggle with the 100L but most of the others I could use it almost like native.
The first thing you notice when you move over are
1 - menus, I know everyone goes on about this, you will too....compare to Canon, it makes no sense, things are scattered that should be grouped together. Spend about 30mins to make your own custom menus with the useful stuff like Format, White balance etc
2 - the size, its not that tall, no place for my pinky to go so I had to buy a little grip to add height
3 - touch screen is turd - I only ever use it to focus when its on the tripod. It does nothing else really.
4 - then you start using it, the AF will blow you away. the entire screen is focus points (almost), assign Eye-Af to a back button (my advice us assign one back button as general focus, and one other back button as eye-af focus). So when there things like cars or static stuff, use the normal back button to focus. or use the normal back button focus mostly but when you have a face, press eye-af and watch it lock on. BOOM! its like cheating. You don't even need to have the AF point on the face, the AF point can be miles away.
5 - how many shots it can take per second. I had to lower it to about M because shooting at 10 FPS just wastes memory card
6 - You will save money in SD cards compare to CF. The sweet spot is 64G - 128G cards. Get 2, shoot dual. UHS-1 is fine because one of them is UHS-1 so that's the fastest you will get. It goes in facing the wrong way...weird
7 - learn to turn off the camera when you change lenses, with it on, there is a static on the sensor (so i've been told). Otherwise you will be cleaning it quite often compare to having the Canon
8 - I like the in camera charging, I use the USB-C socket, you can use micro usb too. The camera doesn't come with a charger but you can get a cheap £12 one on amazon and use your iPhone charger as the brick.
9 - The rear thumb dial I hit it quite often which changes things like bracketing, so keep an eye on it, or you can disable the whole wheel. Unfortunately, there is no sliding lock like the Canon to stop this.
10 - Sony flashes - or all mirrorless flashes do not have IR beam focus assist, however I found Sony's focusing with native lenses good enough to shoot weddings on dance floors so not a problem.
I found weather sealing to be fine. Don't let people scare you.
p..s I have all Godox flashes, great value and just as good as the Canon.