I have both the RF100-400 and the RF 100-500L for my R5 and R10. I previously had the EF 100-400 L mk2, and used it on my R5 for bird photography. My observations are that the EF100-400Lii and the RF 100-500L track beautifully, and focus very accurately. The RF is a little better (not a lot) but more convenient in that it goes to 500mm without needing a teleconverter, and is a little smaller and lighter. The RF 100-400 in my experience is in no way comparable. My first copy was generally soft and tracked poorly, even in good light, and AF often missed focus (even with sky or a very bland backdrop) both for birds in flight and stationary birds. I was very disappointed, so I compared it to my brother's copy, which was definitely better. So I returned mine within the return period. My second copy is much sharper on stationary subjects. It tracks better, but often misses AF especially using eye detect on either body. I also find the bokeh very busy and distracting, blades of grass in front or behind for example are always double. I don't think there is anything actually wrong with the second copy. Of course it is much cheaper than the L lens! However I just don't like it and never pick it up as a result - so I will part exchange it for something more useful. It's very disappointing - especially as I have had good results with previous low cost Canon lenses, for example the EF-S 55-250 STM which was excellent on my 80D and 90D. There are lots of users reporting good results so either they have been lucky or their technique is way better than mine...
