With this compact Z5 Nikon seems to widening the FX niche, thanks to highly affordable FX camera thanks to the form factor of the lighter and more compact MILC. It follows the market for APC/DX systems is now being squeezed even tighter from the top (FX) as well as bottom (phone cameras). The Z5 is a serious product, built around a Z6 sensor with EXPEED6; it meets top industry standards for excellent IQ in lowlight. This is judging from photonstophotos lab testing and my shooting the D780.
The Z5 does not reportedly have the Z6's sensor; both are 24MP but the Z5 sensor is not BSI and the 4K video is with a 1.7x crop whereas the Z6 can do full sensor width resampled 4K without line skipping. For video the 1.7x crop means the kit zoom probably isn't wide enough for a lot of typical uses and a 14-30 or similar may need to be purchased.
However, the Z5 does appear to have a lot of features that are advanced in its class; dual card slots, high-quality EVF, and the ergonomics is consistent with the rest of the Z line with two command dials.
The DX Z50 has the advantage that its kit zoom goes to head-and-shoulders 50mm wheras when used on FX, 50mm framed to make a head-and-shoulders portrait would result in a perspective which would exaggerate the nose compared to the same framing made with classical portrait teles, however, as people are more used to using mobile phones for portraits, perhaps this is becoming more acceptable than it has been in the past. I think the Z50's wide-to-tele zoom is more generally usable than the Z5's wide angle to normal zoom and both are compact. Of course, if mounted on a Z5, the 24-50 may be able to produce the desired result of a head and shoulders portrait in a cropped image, but my guess is that most Z7 users would pick the 24-70/4 or f/2.8. The 24-50 seems to be a good choice for travel and landscape with size, weight and cost as constraints. The portability is achieved by reducing the aperture. I'm actually glad to see Nikon make this lens as otherwise it would seem that FX kit zooms are not really implementing the compactness potential of mirrorless cameras. Hopefully the image quality will be good.