This question is supplemental to another one I've asked about the R10.
If I'm stills only, no video and I only photography things that are not moving, what would I lose by getting an R8 over an R6 Mark II?
I replaced my entire EF FF and EF-M APS-C system last year and made the choice between R10 + R8 or R7 + R6 (or ii). I wanted a compact and lightweight 2x body + 3-4 lens system suitable for general use and (importantly) international travel - since I am 60 and recently retired, so plan to do some traveling. Having the cameras share batteries and chargers was important to me.
For me the two biggest deciders were size/weight and the almighty dollar. This obviously pushed me to R10 & R8.
But in my opinion, and for my uses (which are probably different from many) the IBIS was a nice-to-have (but I figured I wouldn't miss what I have never had and all planned lenses had at least 5 stops of IS anyway, and I don't shoot video), the bigger batteries were also nice-to-have (but since I already had 2x spare LP-E17, got another 2x LP-E17 with the cameras, and bought a further 2x LP-E17 at a discount as part of the "deal", batteries were not of high importance to me).
The dual card slots were of minimal perceived benefit, since I don't sell my photos, and have never owned a camera with dual slots.
The better screens & EVF would have been nice but again, I don't miss what I never had and both R10 & R8 have better EVF than either my M3 add-on or the M5 EVF.
The lack of manual shutter on R8 has never affected me - because I don't have any super fast primes, and if I really need it, R10 has a manual shutter.
The "reduced" weatherproofing (which appears to be a light & fluffy Canon marketing term since there is never any standard or quantification) of R10 / R8 is not an issue to me because I don't make a habit of shooting in the rain, and only one of my new RF lenses is an L lens (though I do still have EF 100-400L ii).
IMO, the important bits, like sensor & AF, and most of the features, are the same (R8 vs R6 ii), so the rest of it is mostly nice-to-have fancy toppings.
So it came down to whether a few extra features (like a C3 mode) and IBIS were worth spending around AU$5900 (from memory) on the R7 + R6ii vs around AU$3200 for R10 + R8, AND carrying around the additional 400g (plus larger batteries & chargers, so probably 500g+) on international trips.
The answer (to me) was obvious - the "extra" AU$2700 paid for RF 24-105L f4 + RF 100-400 + 2x LP-E17 batteries and my back thanks me every time I walk around with the bag for several hours.
But different people have different priorities to mine.
I know about the lack of IBIS but I think that the IS in the RF lenses might be sufficient.
I also know about the lower res viewfinder but having an R10 I know what it looks like and that's not an issue.
What else might there be?
