I don't think the Multi-CAM 3500 is easier to use (I spent countless hours fine tuning cameras using that module to get them to focus fast primes properly, whereas the D5 has been a breeze with most lenses thanks to its auto AF fine tune) but it seems to be that cameras employing Multi-CAM 3500 have programming that has been helpful to some users and that has been changed in the new module. The dynamic area mode on Multi-CAM 3500 cameras seems to switch to and hold onto a closer subject if presented one momentarily, but the system doesn't switch as easily to a subject that is further away, so it has some preference for closer subjects. This can help when the subject is close to a complex detailed background but it makes it more difficult to focus on a subject which is not in the front but behind another subject (think of three runners approaching, if you want to focus on the one in the middle, this would be more difficult using the Multi-CAM 3500 dynamic). I've tried this many times and I could never figure out what the dynamic area modes were doing in the old cameras, until recently there have been these threads where the topic is discussed in depth.
With the Multi-CAM 20k cameras dynamic seems to do what the manual and sports AF technical guide says it does. The tendency to prefer a closer subject seems to have been removed from the programming of dynamic area AF and Nikon recommend the use of group area AF for situations where the subject is small in the frame, to avoid confusion with a complex detailed background, and they recommend the use of dynamic area AF for closer subjects which are large in the frame (in this case the background is less likely to be caught by the camera). This is discussed with quite many examples in the Sports AF technical guide and NPS tips pages. I think also auto area AF can be used to photograph distant subjects against complex backgrounds, in my experience both group area and auto area work reliably in such situations.
When I'm photographing a group of long distance runners, I may have a specific runner that I want to focus on and follow and I would prefer to be able to start shooting as early as possible, using a large aperture to isolate the subject at distance and then take a sequence of shots. For this the old dynamic area AF would not be of help because of its hesitation to focus on a subject behind another; my experience trying to do this with the D810 was frustrating. In the D5 there is no such problem and the camera obeys the photographer and focuses on the subject at and slightly around the primary point, without preference to a closer subject. If you do want closer subject priority the group area AF and auto area AF seem to handle those situations quite well.
Furthermore thanks to the cross-type points in the outermost columns of the Multi-CAM 20k sensor array, focusing on a face in a vertical shot where the subject is placed near the top is reliable with the D5 which it never was with Multi-CAM 3500 series sensors which have all the cross type points concentrated in the middle of the frame. A lot of the time the Multi-CAM 3500 cameras would focus on the hair instead of the face in such situations, even with single point the area of sensitivity was too broad especially for backlit subjects (where the hair was bright and contrasty). The additional cross type points in the Multi-CAM 20k have been the biggest help for me as a lot of my photography involves vertical shots of people and the smaller area of coverage of each point and the cross-type nature of the sensors means I can really focus on the face. I think for users of f/2.8, f/4 and smaller aperture lenses the newer generations of Multi-CAM 3500 (Advanced and Advanced II) have been excellent already but for f/1.4 and f/2 shooting the Multi-CAM 20k really does help a lot.
For DX I think the positioning of the additional cross-type columns is not ideal as they're quite close to the edge of the frame and at least I wouldn't place the face so close to the edge. But there are few complaints about this so I guess users are happy with the broader coverage.
Anyway I think the D7500's AF system choice is as expected, as Nikon likely wanted the D7500 and D500 to be clearly different from each other. Likely Multi-CAM 20k would have made the camera bigger and I think they want to please people who want a compact, less expensive option with still quite high performance.
For those who have problems with dynamic area AF in Multi-CAM 20k cameras, some strategies may help. First, choose group area AF or auto area AF for subjects small in the frame if there is a complex background. If the subject is large in the frame and a complex detailed background not directly behind, dynamic area AF should work if used with single point technique. I typically choose the area size according to my precision in holding the primary point over the subject and the size of the desired target area in the subject (face of the subject in people photography). I think some practice may be necessary to get used to the differences - for me it wasn't difficult once I got the thought "make every effort to hold primary point on the subject's face" in my head.
On Nikon's side, they could provide some assistance in the way of offering a closest-subject priority mode as a custom function option (ON/OFF) in dynamic area AF (to mimic Multi-CAM 3500 dynamic area AF behavior), add 9-point dynamic to the D500 as it is on the D5, and provide better documentation of AF right in the manual with some other examples outside of sports photography (it seems most of the complaints come from bird in flight photographers, so some advice for those photographers on settings to use would probably help).
