The EVF Viewfinder image in the EOS R is very bright, much brighter than the back screen. You can also zoom in with both, there is another cool option when you switch focus to manual it will highlight the outlines (edges) of the image thats in focus, not tried it in low light yet though. I agree with the battery consumption, biggest shock for me, but if doing tripod style landscape work you can get away with keeping the power-save mode on permenantly (problematic for street photography where things happen at the blink of an eye) or switching it off when trekking. One thing that would be nice would be just to have the EVF activate when your eye is on the viewfinder (as in the current mechanism to detect whether to light the viewfinder EVF or the back LED) but not have the back LED turn on at all, BUT to still be able to use it as a touch screen for moving the focus point. Currently can only have the back screen either completely disabled for fully functioning as a display.I don't particularly want to switch from Canon DSLR (EOS 5 Series) to Canon Mirror-less (EOS R Series), but having waited nearly a decade for them to do something that interests me as a landscape guy, I'll probably end up with the rumoured Canon RS (83mp) if it actually happens next year.
I thought I had plenty of patience, skipping the last few models. 5DSR nearly tempted me, but the lack of improved low ISO DR stopped me. 5D IV nearly tempted me, but the res increase seemed barely worth it. 5D III offered nothing I needed. I'm back on a 5DII and still not seeing anything in Canon land worth my money.
There's always the Fuji mini-MF GFX 50R, but it seems barely better than a 5DSR once stopped down, and GFX 100S is to pricey. Nikon D850 - maybe, but starting over with lenses and work flow, is all a bit much. The Sony A7RIII feels like a toy. My canon DSLRs have taken dips in the ocean, been dropped onto concrete, been encrusted in ice, drenched with rain, invaded by desert sands and baking heat, and the worst that's happened is a shutter replacement. So, no, Sony isn't for me. I'll keep waiting for Canon. If they force me to go mirror-less I will.
What I don't understand about mirror-less, like the EOS R, is how I am supposed to compose a pre-dawn image? If it's anything like what "live view" currently shows me, it would be useless compared to simply eyeballing reality through a DSLR view finder. Also the idea that I have to drain precious battery power on multi-day hikes just to visualize possible shots whilst scouting seems like a serious disadvantage.
The only thing about the EOS R that sounds interesting so far, is the idea of mounting a filter inside the EF adaptor for use with TSE lenses. But that seems like a small advantage. Weight? A saving of a couple hundred grams at best. Even then recent RF 15-25 F2.8 is actually heavier than the EF 16-35 F2.8 III, not by much, but still.
Some more F4 RF lenses options might help with weight, but they don't exist yet. The RF 70-200 looks intriguingly small, for example, but I'd prefer a lighter F4 version for hiking.
So, yes, a Canon 5D V would be welcome, if it provided both high res and improved (low ISO) DR for landscapes.