My experience with the 1 Nikon was that the adaptor worked extremely well, although they crippled some of the features. And that was a mirrorless camera, with outstanding focus capabilities. For instance, only the central focusing area was supported. With the new AF-P lenses, I expect the electronic interface will be even better.
You're talking about a budget 2.7x crop camera, which not only doesn't need to be nearly as precise (the available depth of field being a lot wider), but also isn't held to the same standard as a full-frame action camera would be.
When you say a camera has 'good' AF, what do you actually mean? What is it capable of tracking, and what can it not track? What resolution can it focus accurately to, and at what point does misfocus become apparent? And are you applying the same standard to all cameras, or are you applying different standards, based on the class and age of the camera?
Does the Nikon D3 have a good AF system? What about the Canon 5D4? What about the D750? The D3 had a 'good' AF system for its time. But that of the 5D4 and D750 are, objectively, even better. Yet you probably wouldn't regard the latter two as having particularly 'good' AF systems. Why? Because different standards are being applied - the 5D4 and D750, being much newer cameras, are being held to a much higher standard.
Does the 800D have a good AF system? Does the 6D2? Reviews tend to say that the 800D has a good AF system - and it does, compared to its peers. But that of the 6D2 is objectively better, yet the AF system of that camera is often panned as subpar. Why? Because the 6D2, as a higher-tier camera, is being held to a much higher standard.
'Good AF' for a Canon Rebel camera isn't the same as 'good AF' for a 1Dx- or A9-level camera. 'Good AF' for a budget camera was considered to be the ability to quickly and accurately lock onto someone's face for a portrait, or to competently track a slow-moving person. No-one was asking for a 90% hit rate on a moving, dodging, camouflaged subject with the Nikon J5. No-one expected it to accurately track a running dog or erratically-moving child at short range, even with a native lens - and it can't. But that's essentially what is being asked - and should be asked - of top-tier cameras like the A9. And it can meet that standard - but only with a native lens.
And what passes as 'in-focus' for a 2.7x crop sensor shooting at 100mm f/2.8 is likely to be way out of focus with a full-frame sensor shooting at 270mm f/2.8, with a much narrower depth of field (you're not going to get a 100mm f/1.1 lens for the crop sensor, which would call for a similar degree of precision).