I’m not trying to defend Canon or anything, but I would argue that it will be outstanding for stills. I think the R5 has an amazing sensor and I love the images I get out it. Before the R5, I never had the privilege of using IBIS. And I am still not sure if IBIS is really doing anything for me personally because I shoot with the first edition 70-200/2.8 IS (and it’s still phenomenal).
That said, the style of photography I shoot requires me to shoot at at the slowest 1/160, so perhaps I’m not the target use case for IBIS. But as a stills camera, I believe the R5, even if it didn’t have IBIS, is amazing as a photo camera purely on the ease of use, AF, and sensor.
I think if the use case requires really slow shutter speeds, handheld, then you could argue that it’s only adequate—but then again, were the 1DXiii, 5Div, and R only adequate stills cameras?
I agree - the original R5 (which I own) would still be a superb camera if it didn't have IBIS. I also own a 5DMkiv and previously had a 5DS, so I'm able to make a direct subjective comparison of the stabilisation of each of these cameras.
With the R5, if you're using RF lenses in which the OIS synchronises with the IBIS, I'd estimate at least a 3 stop advantage in stabilisation, compared with using OIS stabilised EF lenses on the 5DS. That makes a huge difference in terms of being able to use slower shutter speeds and/or lower ISO, for static subjects. With moving subjects, it allows you to use smaller apertures for more depth of field, if you need it.
But the biggest advantage of IBIS is that it enables you to much more easily hand-hold non-stabilised lenses (of which I have 3). IBIS alone provides an extra 1.5-2 stops of "handholdability" with my EF 180mm F3.5L at close distances (1:2 to 1:4)
Another advantage is that IBIS allows you to stabilise completely manual lenses from Laowa, Zeiss, Samyang etc.
For context, the lenses I use include RF 24-105mm F4L, RF 800mm F11, EF 180mm F3.5L, EF 100mm F2.8L macro, T/S-E 24mm F3.5L, EF 100-400mm F4-5.6L Mkii, and Laowa 25mm F2.8 Ultra-Macro. Obviuosly the effect of IBIS varies according to which lens is used.
Undoubtedly the R5C is a much superior camera to the R5 for videography, but the camera's digital stabilisation is non-functional with stills photography, so in terms of usable shutter speeds, the R5C is probably on par with the 5DS (which is OK, but certainly not a patch on the superb OIS/IBIS of the R5).