Is there any benefit to have a wired backhaul for an Orbi Mesh system?

I'll throw my .02 in as I moved from wireless to wired backhaul...

First, there are some layout differences imho. With a wireless backhaul, you need to place any satellite(s) in an area with a decently GOOD signal, otherwise there will be sync issues. And at the same time, it should be in an area that currently is near to area(s) with poor WiFi coverage - that's the whole purpose of mesh, to extend as strong a signal as possible to the whole house. Now, it can be challenging accomplishing both of these requirements and still having an acceptable place to put the satellite aesthetically in the house.

Now it's totally different with wired backhaul. There, you find the WORST area of signal in the house that you can still get an ethernet cable to! Because you don't have to worry about relying on a strong wifi signal for syncing. And this will spread the satellite's wifi signal to areas of the house that most need it.

So last, how do you do this, how do you find the signal strength? I would highly recommend to do this right, you need a software app that shows real time signal. There are a number of free ones. I use a Mac laptop, and walk around the house running either/both of 2 tools - WiFi Explorer Lite and iAnalyzeWiFi. Both are free and in the Mac App store. I am sure there are comparable tools in the Windows world, if not more of them. Both tools will give a real time view of ALL signals in your house at that spot; see the picture below. As a bonus, you'll even see neighbor's signals if they are close enough, so it gets really easy to determine channels to set up with the NG routers for 2.4GHz and 5Ghz so that contention between networks is minimized.

So here is the most reliable process imho, whether you will use wired or wireless backhaul satellites. Set up just your router. Next, if you have some capability to use wired backhaul, set up those satellite(s) first - walk around the house in the areas where wired backhaul is available, and find a mix of the worst wifi signal area, likely furthest from the router or behind some obstruction, etc., maybe it's somewhere perfectly positioned to split the coverage in the house etc., and place the satellite there. Don't be afraid to experiment. After the 1st satellite is installed, go walk around again with your signal app - see where the signal coverage issues are. Move the satellite to another area, and see what happens to the weak signal areas. Only go to the second, or if needed third, satellite if it's really needed after exhausting all the coverage options and picking the best one - and having a good idea where you are going to place the next satellite to cover a big area not covered by the router or satellite 1. In my case I learned how easy it becomes to figure out where to place satellites doing this!

Now, in my case, I have an old 4800 square foot house, with tall ceilings, with 3 floors plus a basement. My cable comes in through the basement, I can't easily change that. So that's where I started this exercise. Before I figured out this smarter way of doing it, I had 1 router and 3 satellites - one on each upper floor. I put them where my wife wouldn't be mad at looking at them lol, but "spread out" in what I thought was logical. But it didn't run so great - I saw a lot of what I see on the forums - performance problems, signal problems, devices locked on to the wrong router/satellite, etc etc etc. Well, after I did the placement with my approach from above, guess what - I discovered that I had too many satellites! I ended up having full house coverage at stronger signals than before, with only 2 satellites - one on the opposite side of the house on 1st floor, and one back on the router side of the house on the third floor. Who woulda thought - the whole second floor, 12 foot ceilings, probably 1000+ sq foot, has no satellite! And it works perfectly - depending on the side of the house, it gets super strong signal either from the first or third floor satellites. The coverage is fantastic and I have never had a problem like I did before. I realized also that this process exposed the fallacy of another assumption I see people have on these forums - "more satellites have to be better than less" - wrong! Signal overlap will kill performance and confuse devices.

The key is to be patient, take your time in mapping out/learning where signal strengths are high/low in your house. If you take the time to do this part right, you may be surprised in needing less satellites, you won't have signal overlaps that the devices can't handle, and performance will be the best it can be. But whatever the number needed, the satellites will be placed optimally.

One last note from the OP's post - it seemed to me you expect the same wifi performance from all devices from the same spot - that won't happen. Each device has different hardware in it. A 2022 iPhone is faster than a 2020 iPad; and so on. I have had devices sitting next to each other running speed tests and saw different results. So don't get distracted by that.

Good luck!