Electronic shutter 'is certainly an 'exciting' new feature in the Z9, but there's more to consider in the very high frame rates and especially the breakthrough advances in the autofocus. This is embodied in the 3D Tracking with universal Object Recognition.
The ability of the Z9 to recognize objects automatically and simultaneously has profound implications in changing how photographers work in the true sense of the art. In a Live
Discussion earlier week (link etc above in this thread) [after ~32:00 ], Thom Hogan emphasized how the AF is structured to work hierarchically.
Moreover, judging from preliminary evidence of images and EVF footage it is already clear that if 3D tracking cannot find Eyes, then it reverts "up" to Heads; then "up" to larger Bodies/Torsos.... When this AF mode is activated, it's always tracking shapes. Nevertheless, it's not as yet at all clear how Nikon has actually implemented this hierarchical shape recognition are not at all clear.
It is also very clear this AI empowered merging of 3D Tracking with Object Recognition works very reliably, seeing how it very quickly grabs on to any subject - among a diverse range of active subjects: i.e humans, animals (including fishes), birds and vehicles! And the Z9 tracks vehicles analogously to how it classifies human and animals hierarchically thus Vehicles > Cockpits > Front-of-a-Vehicle, down to smallest high contrast objects, namely helmets, headlights.
As with Joe McNally and other highly experienced Pro's, with high frame fps and truly Flicker free EVF (freed from conflicting against demands of high frame rates etc) TH concludes the Z9 new auto/3D AI tracking frees up photographers to concentrate on optimal image faming, composition etc.. Bottom line.... there are all considered (and compared so) not a lot of limits to the Z9.