Top issues with Nikon UI:
1. Bracketing. No one button bracketing. The number of control steps required to set up bracketing is ridiculous. And, you can't set release mode in a User Mode, so that doesn't help. Pansonic does it best, by connecting release mode to the bracketing setup. Olympus has a one button mode, and with the OM-1 it now uses e-shutter.
2. Custom (User) modes. First, Nikon has two approaches: User Modes and Banks. I always wind up with Nikon bodies that use each, and they are worlds apart. Panasonic tends to have 5 Custom Modes (great), but the same limitation as Nikon in that you can't recall the settings. Olympus does it best; you can recall the settings, determine whether the settings are static (Nikon User Mode) or dynamic (Nikon Banks) for each setting, and use a function button to activate the setting (much faster than using the dial).
3. Speaking of Custom Modes, the only way to access custom modes with Nikon is on the mode dial, and that is on the left. I can't switch custom modes while keeping the camera up to my eye and holding the lens stable.
3. Arrow Pad vs Joystick. Redundant functionality, both Olympus and Panasonic allow assignment of different functions to the Arrow Pad. Panasonic is even more flexible than Olympus. But Olympus is more flexible at assignment to labeled function buttons. This gives Olympus and Panasonic a lot more assignable function buttons than Nikon.
4. Image review. The way Nikon displays review images is jarring and time consuming. After taking an image, Nikon resumes live view and then brings up the review image. This is slow, and I really dislike having live view resume before the review image is displayed.
5. Highlight/Shadow in Live View (also called Zebras when just looking at highlights). With Olympus and Pansonic, I can see where highlights will be blown, and either adjust EC or switch to bracketing to deal with it. With Nikon, you have to take the picture and review it. Waste of time and loss of opportunity. I have created a custom profile that blacks out blown highlights, but that ruins JPG images.
6. My Menu. The OM-1 has killed it with their implementation in the OM1. So much so, that I ruled out upgrading to the OM-5 because they didn't do the new menu system. I can stuff it with all the major settings I use. Nikon is a big flat list with settings and organization controls combined, which means I wind up scrolling a ton to get to what I want.
And, just for kicks, the Nikon Z9/Z8 have a version of Pro Capture called Pre-Release, but it is JPG only! Which makes it a toy mode in my opinion.
Not saying there aren't things I really like.
+ I really like that Nikon flags menu items that have been changed from the default. I like that Nikon can save settings to a card (as can Panasonic) but they should allow selection of which card in the save/load settings function. One of the reasons I upgraded my Z7 to a Z7ii was to use SD cards to store settings.
+ I like that Nikon kept the on/off switch and menu button on the right, and wish Olympus had the same design. But then, they left the playback button on the left, ignoring the fact that the best way to playback images is using the EVF. Same problem with Panasonic.
Basically, I don't like left-hand controls because my left hand is holding the lens and my eye is watching my subject or using the EVF to see the controls. Olympus also put AF selection and drive mode on the left, but I can access them on the right using function buttons so I never use the ones on the left. And my one big complaint about Olympus is that I can't access Menu on the right.
This is one of my main shooting scenarios: I'm shooting a bird in a tree using my BOB settings (Bird On Branch). Basically slow shutter, slow continuous or single shot, small AF point, maybe subject detection, maybe manual focus. Then I want to quicky switch to my BIF settings (Bird in Flight). Fast shutter, high speed continuous, Pro Capture, large AF area, subject detection. I can do this with OM1 with the press of a button and without having to take my eye from the EVF or left hand from the lens and lose track of where the bird is. And switch back if the bird doesn't look like it's ready for flight. For me, this scenario overrides any other considerations.