Let me start off with a straw man: static scene autofocus with mirrorless cameras is more accurate and more consistent than with DSLRs.
That indeed has been my long-term experience with virtually all the mirrorless cameras (versus almost all the DSLRs). Even though some older mirrorless cameras may be a bit sluggish to get to the focus point (e.g. the X-T100), getting accurate and consistent focus on landscape, still life, basic travel scenes, and posed subjects is something that every one of the mirrorless cameras achieve quite well these days. And it's gotten better in the fifteen years of producing them to the point where static subject focus should be excellent on anything you buy.
Where autofocus is still a contentious subject is with action.
And here's the reason why: most photographers struggle with keeping reasonable and steady framing on fast and erratic action; they have no extra brain cells or ability to also control and direct an autofocus system at the same time that they're struggling with maintaining their composition. They tend towards "all automatic" focus modes for action. It's those "all automatic" uses that produce so much of the differing opinions you see elsewhere.
Remember, even the best of the mirrorless cameras are not showing the "live moment" in the viewfinder as in the DSLR: mirrorless viewfinders are always lagged. In many recent cameras, such as the Canon R1/R3, Sony A1/A9, and Nikon Z8/Z9, that lag is so minimal as to be ignorable, plus the viewfinder never blacks out. It's easier to follow action with those cameras.
In other cameras, that display lag and blackout can be significant and problematic, so when you start trying to take bursts of fast moving and erratic subjects your ability to keep the camera composed and focus sensors consistent on objects in the scene becomes quite troublesome. Most people already have the problem of maintaining composition on erratic action, even on DSLRs. Thus, compounding the problem with a lagged and blacked-out mirrorless display just makes everything break down faster.
That's why subject detection became so effective for many. In effect, the camera is trying to promise you that you can keep your brain centered on composition and the camera will magically work out the focus. The problem that many discover, however, is that if you leave everything to the camera, it often doesn't exactly nail the focus plane to where you think it should have been. Sony cameras have a tendency to drift the focus plane slightly while tracking action, though the results are still quite good. They're just not perfect, and when I or others point that out, the bickering over what is happening begins.
Things get worse when you're using a camera that doesn't have the latest subject tracking mechanisms. Those cameras generally require more input from you to get great results.
I've struggled with trying to describe how it is that I'm getting rock solid results for very tough action sequences with my Z6II and Z7II while others aren't, for example. What it boils down to is attention to details and practice.
Simple question: did I prefocus, did I let the camera find the mountain lion and focus, or did I do something else? Answer: all of the above.
The details come in learning what your camera's autofocus system does and doesn't do. In what settings you can make prior to taking an image that might help. In understanding what the camera is actually doing. In eradicating things the camera is doing that get in its (and your) way. In using the right lenses (the newest Z mount and older AF-P lenses tend to do a tiny bit better than the oldest AF-S lenses on the Z cameras, for instance).
The practice comes in just working (and sometimes experimenting) until you get good and consistent results. For me, that's been tens of thousands of images using all the Z's.
Thing is, getting good autofocused results on erratic and fast action is possible on most modern mirrorless cameras (and DSLRs). However, not everyone is willing to take the time to understand the details of their camera, let alone practice enough to get the best possible results consistently.
I will give you a clue, though: the goto advice for any Canon, Fujifilm, Nikon, or Sony user who's struggling with "all automatic" focus is to take a little more control and force the camera look at a smaller area. That pretty much always improves the focus you're getting. The tricky part is that this only works as well as you exert control. It's not a button push and you're done. You now have to more actively instruct the camera rather than it doing all the work.
The casual shooter that's only using their camera once a month or less simply wants magic.
We've moved closer to true focus magic over the years, but it's still not completely there yet, even though Canon, Nikon, and Sony are all delivering some reasonable approximation pretty far down their lineups now. Meanwhile, a lot of commentators and reviewers aren't spending enough time with every camera that comes down the pike to really say that they've done the detail and practice work. It doesn't help that the camera makers are closed lip about what the camera is actually doing and offer little help.
For example, when Nikon launched the Z7 with great fanfare, not a single Nikon product manager or employee I talked to could describe what the autofocus system was actually doing (they still really can't, by the way, and I still have at least three unanswered questions that only a Nikon design engineer is likely to be able to answer). Every Nikon Ambassador I encountered in those first demonstrations was using AF-S (single servo focus) because they weren't getting the knowledge (or time) to master the continuous autofocus system. When you hand cameras to people who've never seen or used it before with no instruction, what do you think happens?
Most of the camera companies, bless their souls, have a longish history of iteration now with their mirrorless focus systems. A lot of people don't remember just how bad the original Fujifilm or Sony autofocus were for continuous action. Far worse than where Nikon was with their original Z6 and Z7. But Sony is over a dozen focus system updates across the Alpha line. Nikon improved the Z9—which was already really good when it first appeared—four additional times.
Even with all that iteration, I'm still not sure we're yet at the point where anyone can truly claim "just pay attention to the framing and the camera will do the rest." Though with the Canon R1/R3, Nikon Z8/Z9, and Sony A1 and A9 Mark III we're getting close.
And thus autofocus remains a highly contentious subject.
It's time for another straw man: with almost no exception, I'd rather have the current top-end mirrorless autofocus system from any vendor rather than any DSLR autofocus system I was using a dozen years ago.
Put another way: we're far better off than we were not too long ago. And every camera maker is working frantically at pushing things even further into the good.
Stop arguing and do more study and image taking. Amazingly, the autofocus system you're complaining about will get better when you do. /mic drop
