The question about whether 3rd party EF-to-RF adapters are good enough seems to come up a lot with new people and those with limited budgets. Just documenting my experience with the Meike and Vello adapters compared to the Canon adapter. Both of the 3rd party adapters have faux internal design which looks to mimic Canon's adapter. Both of the adapters have problems in actual usage when it comes to internal reflections. The Canon does a really good job of dampening internal reflections. The 3rd party adapters not quite as good. So while the 3rd party adapters are good enough to pass the electronics, in slightly more demanding light conditions they have problems.
I recorded videos that make it easier to see when you get flair with each adapter in place. Both of the 3rd parties exhibit secondary reflection because of the lack of REAL light dampening inside the adapter's tube.
Youtube video of comparison EF-to-RF adapters
Below is what the test setup looks like. The Godox AD400Pro LED modelling lamp on a stand is used to simulate a bright light for the sake of reproducibility. For video using the R6 on a tripod and Canon EF 85mm/1.4 IS with lens hood.
Just posting for knowledge because I used the 3rd party adapters for about a year with this flaring issue before the Canon were readily available.