Motorola's Z was one of two Android phones that made modular expansion their primary selling point. Neither the Z nor the LG G5 achieved notable success. When it came time to replace the G5 LG abandoned modularity for the G6.
Motorola has stuck with its plan and the Z2 utilises the same mods as its predecessor. That's probably a mistake.
The problem with these modular phones is one of continuity. Those LG G5 buyers are stumped when it comes to an upgrade, that investment in 'Friends' (as much as they were available) is gone. Whilst Moto Z users have an upgrade path to the Z2 which retains the value of the Mods, if the Z2 is also a commercial failure who is to say whether there will be a Z3?
Without a cast iron guarantee of future support, or a standard for add-ons which transcends Motorola's proprietary technology, a punt on a Z2 is laden with financial risk.
Then there's the mods themselves. Neither the Hasselblad nor JBL add-ons are a particular convenient or cheaper than buying a discrete compact camera or Bluetooth speaker. Both of which offer the advantage of having their own battery and, most importantly, won't be rendered worthless if your next purchase isn't another phone with Moto Mods. Other Mods are just plain expensive.
And that's the big problem for the Moto Z2 range. Without the USP of the Mods it really doesn't have any reason to exist. Or for you to buy one.