It would be most useful to find someone who has worked on them.
I suspect many users like myself have only used a couple or maybe three models over the years.
Conventional opinion is that the R6 and R6.2 are most reliable because they have mechanical shutter firing.
I have used an R4s, a R-E ( neither of which I used a lot ) and two R7's, which I did use a lot. One of my R7's is now unreliable after 25 years, the shutter will not fire unless the metering selector is moved back & forth a few times. I have not investifated what causes this, because it's hard to find anyone who will work on 'R' bodies now.
There are lots of nice R bodies for sale now, some at very attractive prices, many are very lightly-used.
However i don't know if all R's break down from hours of usage, or whether deterioration of electronics can make them unreliable after a long time on the shelf.
I will be interested to know myself.
ps my R4s body is over 35 years old and still works fine.