I think that the OP's post certainly sparked a lot of good comments, perspectives, and constructive counter-points.
Some of the discussion centers on AF performance. In recent years, it seems that the Pentax brand has been criticized for its
relatively poor AF capability -- not by all users, but in notable numbers.
AF on Static Subjects. Concerning the present thread, I am having trouble understanding the problems that the OP has had in focusing on static or slow-moving subjects. I think it's reasonable to assume that the camera(s) were calibrated for AF Fine Adjust and that the problem camera was checked out for possible damage or component failure. The only comparison I can make directly is to my own experience -- I'm not an 'action shooter', and I've had very little issue focusing on stationery targets. But I'm a sample of one. It would be useful to know more about the conditions under which the OP had those somewhat surprising problem shoots.
Pentax compared to others. Concerning the AF performance of Pentax compared to other brands, it seems that most conclusions I have read elsewhere are based on hearsay or anecdotes. I do not refute that there are problems. However, despite my daily monitoring and reading of numerous camera/photo web sites during a number of years, I have
never seen a credible AF test of recent Pentax cameras compared to other brands. By credible, I mean rigorous, well-designed and executed tests that are repeatable under controlled conditions and provide numerical data. I exclude the random wobbly-bicycle runs that are conducted variously in an alley or along a sidewalk.
Nikon issues. Finally, concerning the Nikon system, I understand that the D850 and many of the Nikon lenses are fine pieces of kit, indeed. However, the D850 is not without problems. A quick scan on the DPR forums reveals all sorts of issues: LV AF hang-ups; AF calibration; occasional freezes; AF release priority problems; "Autofocus Blues"; inconsistent focusing (requiring repair shimming); AF problems with third party lenses; "AF stops working"; "Low AF hit rate", and 3-D AF tracking failures.
I don't mean this post as a slam against the OP or to try to convince him to stay with Pentax. I think he'll enjoy the Nikon system, and wish the best.
Last edited by c.a.m; 01-17-2019 at 08:19 PM.