There are a couple of head-to-head comparison videos out there. I will link to one (or two) below.
I think that pretty much any and every ND filter is going to have color / tint shifts. I have been using a three-stop Hoya MC (HMC) ND filter and it definitely results in a blue temp / magenta tint shift. I always have to adjust my white balance slightly amber and slightly green. That's a filter that costs amount $45 for a 77mm size, so it is what I would guess you would call "mid-tier" in terms of pricing.
In doesn't seem to affect sharpness in any noticeable way.
Haven't noticed any vignette with the filter and there isn't too much added flare, but there is some increased blue ghosting when the sun is in frame or just out of frame.
I use it outdoors when shooting in LOG (which requires ISO 640) so that I can open the aperture from f/22(where diffraction would be heavy) to f/8 (the sharpest aperture for my lens).
Three stops isn't all that heavy of an ND, so maybe others who have shot with heavier ND filters will provide better information for you.
Slightly Off Topic: For cameras that DON'T have a built in IR filter (or have a very weak one) you will want to get an ND filter with IR filtering, too. I know that Tiffen makes some. I am not sure what the R6 IR filter situation is. It might be great (most likely it is).
I based my choice of the Hoya MC based on Griffin Hammond's video directly below.
Also, I just watched the slanted lens review of some of the more expensive variable ND filters (Polar Pro, Syrup, B+W, and Tiffen) and their conclusion was that the best variable ND was to... use a couple of non-variable ND filters stacked. In THEIR conclusion, cheap Amazon single-density ND filters looked better than the most expensive variable ND filters.