I shot with the EF 100-400mm L mkii on the 6dii and the AF struggled with slightly higher aperture. I later used it on the EOS R and it worked flawlessly even down to f22. So yeah, I agree with the sentiment. In my experience the same can be said about the EF 16-35mm F4 L, 70-300mm and EF 70-200mm F4 L. Those are the only lenses with which I had a direct comparison. (rented the 6dii for testing it before deciding to purchase the EOS R instead).
AF expectation has gone up dramatically with the mirrorless / software based Af systems. Photographers can now do more, have better accuracy and way better tracking. The old chip based AF is slow and pretty much redundant when compared to what even a EOS RP or R8 can do.
I personally don't own any RF glass, my entire lens catalogue (which is quite vast) are all EF lenses. I tried a EOS R when it was launched and didn't like it, it felt too much like a beta camera. When the R5 came out, i tried it and loved it, but it was too expensive and I didn't need 45mp. The R6 was a bit too low res for me to make the jump from my trusty and proven Eos 5Diii's. I bought a R8 just after launch to dip my toe into the newer Canon ecology and was blown away by how far Canon had come with their camera tech. I soon wanted a better version of the R8 and bought a R6ii as my primary camera and my R8 was my second camera. Only recently have I upgraded my R8 to a R6iii (I dallied with a R5 for 6 months, but that's another story).
Most of the RF glass is a wee bit better in some regards to the EF glass. Some of these benefits are marginal and others are quite dramatic. However, if you already have great EF glass and it's giving you great results (and it's paid for) then it's free and doesn't cost you anything. Where as the RF lenses are expensive for sometime very little benefits for the cost.
For example, the EF 100-400 f5.6 LIS II vs the RF 100-500mm f7.1 LIS. Sure the newer lens is slightly lighter, longer and it's AF is a bit better too. However optically, the old one is brighter and equally sharp. In the UK, the nwer lens retails at around £2.5K, the trade in for a the EF lens is often quite low, maybe £800 in trade in. So I would still need to find £1.7K, which is a LOT for something that's only a bit better than my existing lens.
Other lenses like the RF 50mm f1.4 VCM L and the RF 24mm f1.4 VCM L have no direct EF lens comparision that is worth mentioning.
Some lenses like the EF 11-24mm f4 L are amazing performers and gain the option of using drop in filters behind the lens when using a EF to R adapter. This means that a large and bulbous lens like this can use filters very efficiently when crossing the EF to R mount divide. It's RF equivelent is the sublime RF 10-20mm L. Which is a stunning little lens. It's a little wider, not as long and it looses the drop in adapter option (obviously). But it is so much smaller and easier to use than the giant EF version. Sure, the EF version doesn't rely on corner stretching / math geometric correction, but compare the size difference and it's a night and day experiance. As you can see, you gain some features and loose some others. it's not quite a like for like comparison.
