I have an A6400 but bought it for different reasons. I wanted a smaller, cheaper camera for family holidays etc. as I prefer not to risk my work stuff for this just because of the hassle, if it was stolen or damaged.
I have A7III's for work and had planned on using one of those for dedicated photo trips.
Since buying the A6400 though my mind has changed and it will be used for all of my photos trips and my family hols and when I am shooting something for myself it is often the camera I reach for first. The a.f on the A6400 is just superb, I had an A9 on loan for a month or so some time ago and in my personal opinion I would say the A6400 is better. That was before the recent A9 firmware update though. The A9 with the new firmware update is probably better, but I haven't used one. A.F performance is better than my A7III's and I am very happy with those. The smaller camera and smaller A.P.S.C lenses are great for travelling, I mostly use mine with the Sigma 30mm f/1.4. The longest lens I have used with the A6400 is my 70-200 f/4 and it works great although the ergonomics of using a small body on a longer lens took a little getting used too.
The main disadvantages for me with the A6400, I already knew about before I bought it. It only has one control wheel and it doesn't have a joystick which I personally prefer to using the touchscreen or the buttons on the back to move the focus point. It doesn't have ibis either but I haven't really noticed that as being a problem. It also only has the one card slot although for my personal stuff I am not concerned about that. It also has the older, smaller battery which I was very concerned about before I bought it. Although I haven't actually found that to be much of an issue, battery life is better than I thought it would be.
I have taken it to 2 weddings with me, the first time I hated using it, I found it slower to use in terms of being able to change settings etc. Since then and after getting to know the camera a bit better I have taken it to another wedding mainly just to use with my 70-200 f/4 for an outdoor ceremony where I wanted to stay a bit more out of the way and it worked great. Due to the single card slot though it wouldn't be something I would use on a regular basis.
I am very happy with mine. If I woke up tomorrow morning and decided to stop photography as a career. I would happily sell of all of my A7III's and associated bits and be quite happy with just the A6400, my A.P.S.C lenses and my 70-200 f/4.