At the same time as I was trying the EF 70-300L with my R5, I also took along (with quite a bit of juggling) an R6 MkI, and the EF 100-400L Mkii, a fabulous optic from the DSLR days, and one I occasionally use with my 5DsR body.
The methodology was just the same - to torture test the combination with a range of focusing scenarios to see how the combination would work and if there was an issue as far a EF lenses on R bodies was apparent.
Spoiler Alert: It wasn't - in fact the system worked extremely well. Now, I appreciate that the sensor is smaller in MP count, but the focusing system was the same as the R5 which was released at the same time, so I think this is a reasonably valid experiment. For many people who have the EF 100-400 MkII, the results might be helpful in deciding whether to move to an R body with this and other legacy EF optics. I recognize that I have chosen L series lenses, but that is what I have, and a couple of posts concerned at L lens performance on the R bodies started this, so for me this is valid.
As before, all shots taken hand-held, available light. If anything, the keeper rate was greater - no fumbling this time and the smaller sensor FF capacity of the R6 seems to have about 1EV DR more than the R5, which was helpful in low light.
The first image is as shot, the second image of the Lion is cropped significantly to show focus on the eye.
So, again, you are welcome to ask questions and draw your own conclusions, but for me this confirms that these legacy EF lenses work fine with the R-series bodies, using the Canon EF-RF adapters.
The cropped photos are a challenge to me for certain photos of wildlife. We have a number of birds that frequent our backyard. Will keep this "technique" in mind.
I have the same equipment. R5 and Ef 100-400mm V2 . There is a photo of a bird I took with the extra 1.4x extender attached. With this combination, the bird's eye tracking is stable and the image quality is extremely good.
I am impressed with the quality of the image, given the use of the extender. I had the 1.4 and 2.0 MkIII versions and was not a huge fan, so I have put them up for sale. The on-going success that you and others are having with legacy EF lenses reinforces my belief that the perceived incompatibility of EF lenses with R-series bodies is not systemic, but likely individual hardware, configuration or operation.
I have several Ef lenses. Canon Ef 70-200mm f/2.8L IS USM II, Canon Ef 300mm f/2.8L IS USM II, Canon Ef 100-400mm f/4.5-5.6L IS USM II . Also TC 1.4x III and 2.X III. All this works seamlessly with the Canon R5. I am happy with their performance. Maybe I have good copies.
Honestly, I don't think there is that much variation between individual units to mean that a lens will or will not focus. We are all seeming to express the same experience, that there is no systemic reason not to use legacy EF lenses with the new R-series bodies - use and configuration aside.
I love these open, honest conversations! They're so helpful! Many photographers have told me that for them (even the handful of Explorers of Light I've spoken with), more often than not, adapted EF lenses work better on their EOS R bodies because of the advanced focusing system and advancements in image processing. Take from that what you will.
In comparing the performance of an EF lens on a DSLR vs the same lens on an R-series body - although I have only got R6, R6 and R6II units, I would agree with them. That is why I am retaining a fairly significant number of legacy lenses, including a couple of Sigma units, that still work fine for me when shooting wildlife.
I think there is a tendancy for people new to the R system to not do their homework and understand the nuances of the new focusing system in particular - it is quite complex and for one to get the most out of it, then one puts some time into it. The old adage of 'read the manual' is still valid - although I have observed plenty of You Tube videos on how to set up R-series bodies for various genres, so there is a lot of info about, if one chooses to look for it. For some the fact that their system has issues must mean that the issue is systemic across all units, which is drawing a very long bow, but neatly removes the responsibility from the user and dumps it on the provider.
In NZ we have a saying describing an outstanding or dominant performer being the target of abuse and angst, it's called the "Tall Poppy Syndrome". I believe that Canon, as the dominant player in the market falls under that. I continue to be bemused by people who demand that Canon (or any other camera maker) adapt their cameras to work with 3rd party lenses and vent furiously over it. I see a lot of posts on this forum to that effect, and much of it is associated with other, unconnected frustrations that reflect how damaged society has become over the last few years: there is a lot of anger out there, sad to say...