I don't mean to sound like Sony is the best either. I've just not had the negatives, and have suggested a few products along the years, and everything has turned out well. Like I said, I did have a Canon in there, well, still do, it still works. Kodak was my first actual one, took great pictures for a starter.
I don't generally go looking for negatives ~exactly. With most products, I know that it's a fact, more people will complain and report problems more than they will praise them, so results a person sees are skewed. Of course, like I said, if you see an issue mentioned repeatedly, then it needs looked into. Higher dollar item will get more scrutiny. But, an occasional ~something every once in a while is not always a concern, any brand can have an issue, or even a bad model, doesn't mean they are junk.
Taking into account they are not full frames, and the nature of their native lens availability (and possibility of adapters), the little a series are well respected, at least when I was researching. And the prices they still command today, shows people are willing to pay for them (just a few months ago was the first real dip I saw in pricing after all these years). I was thinking of getting a newer one with in camera stabilization, would make it a ~bit handier in some situations. But, other than thinking about it, not ~really a reason to switch.
What you're looking at, at the price, with my limited looking at it, I would probably also choose the d850, really. For video I like the p&s (the a5100 does great though), the zoom is more seamless and again the ease of quickly picking it up, taking it along, and generally it's things like wildlife, not close. Yes, not the quality, but "good".
Warranty business would be a concern. Of course, some credit cards automatically extend them for free and have their own process. And, although sometimes an extended warranty is a gimmick, if not too bad of a price, they can actually be worth their weight in gold, if a reputable company. I can't say that I saw a lot of issues back when I was looking, perhaps it's changed.
I don't know, when I look at about anything, I see more of the Ford vs. Chevy, IH vs Deere argument. You might have a worse transmission in one model, better engine in another, better location of controls in one, and in the next model year it might be the reverse. In the end, they all have "ok" models.
Another easily found random comparison
https://www.lightandmatter.org/2017...y-a7r-iii-vs-nikon-d850-which-should-you-buy/
Renting? I've seen people do that, didn't look if that works online. I'm pretty sure there isn't a store that does that within probably 180 miles around here.
Anyway, I have a cow ranch, not a little one, not a big one, but it's what I do. I'm not a pro photographer, so probably it would be best not to pay any attention to what I say....