I've have just over 600kms on my 18 1000.
Problems so far.
Starter relay stuck so bike wouldn't start=easy fix
Coolant leaking out of drain bolt when bike sits for extended period=add coolant once in a while
Rad plugs with mud easily and bike runs hot
Brakes squeak like crazy
Have a heat shield or something rubbing on exhaust=investigating shortly
Passenger seat belt won't lock
Doesn't start well in cold weather
Other than those few minor issues the bike has been awesome. I added a 2" lift which raised the bike closer to 3" and that really helped it get through the mud easier.
Doing the first service on it here shortly so I may find some more things but so far so good.

Seems like almost all CFM brakes squeak. I cured mine by painting Disk Brake Quiet on the backs of the pads. Quiet ever since.

The little bottle in the middle.

I have the same machine, my passenger harness wouldn't buckle either but my dealer swapped it out on the spot when I mentioned it (They actually pulled one out of a 2019 1000 in the showroom when I brought my harness in to them).  I also had rattling heat shields, I solved it by getting better hose clamps and tossing the shitty oem ones holding the shields in place.  No problem since.  Had to do it on both front and rear pipes. 

And as for the cold weather thing, when it's cold out I turn the key to let the fuel pump prime, then turn off the key and repeat a couple times then start it up.  Seems like it always takes 3 pump primes for it to start right up when it's colder out.  It's fine in the warmer weather though.

Went for a decent rip last weekend and ended up in some nasty mud. This thing went through everything except one hole where my front tire popped off the bead. The lift kit really helps this thing get through the nasty stuff. One thing that I didn't like was the stupid fender flares that kept falling off once the mud was built up on them. I'm going to source some Dzus pins or something to hold them on better.
What do you guys run your front tire pressure at? I popped both fronts off on that rip. I'm also going to chop the cage. It's way too tall and I think it makes the bike look goofy.

Here's a little video with a few shots of the Z.
https://www.youtube.com/watch?v=P82yleN4PE8&feature=youtu.be&fbclid=IwAR0C8jBYTGlWw7d9BDrvY__LV0-po4BqSz5z7FxAWm4dVwqhqFQKnEyjqIg

I'm at 1600 mi on my 19 Z1000. I have only had 2 major issues and couple minor electrical problems.

When I first got it I would drive like 10 mi or so and all 4 brakes would lock up. Took to dealer they replaced the master cylinder and all four rotors and pads.

Then at about 1300-1400 mi I grenaded the rear diff. They have no clue why it happened but all of it was covered under warranty again. At same time I was having a main power issue that turned out to be corrosion on the main fuse under the seat.

Last week I ended up with a fuse blowing problem which ended up being the license plate wire shorting out.

Overall I have to say this machine has impressed me and other people I ride with. I have put in 3" lift in it and it goes places that Can-Am Mavericks can't go. I work and play at a local Mud Facility and I have been the guinea pig to blaze some new trails there and it has done great.