I'm pretty well locked into the Nikon system, so the choices for me are the Z7 or Z6. At this point it's a toe in the water for mirrorless because I'm not planning to make wholesale changes to my existing F-mount lenses. So I picked up a Z6 in late November and probably have 4000 frames with the Z6 and various combinations.
I would have chosen the Z7, but I already have the D850 providing a high resolution camera. What I really needed was a full frame medium sized sensor. The cameras are quite similar in other respects, and the Z6 makes a great camerae for events, travel, golf, and many types of wildlife photography.
Image wise, the kit 24-70 f/4 lens is solid and is as good or better than my 24-70 f/2.8 AF-S lens for the F-mount. I've used the FTZ adapter with all types of lenses from the 14-24 f/2.8 to the 300mm f/4 AF-S with teleconverter or the 600 f/4 AFS VR. The FTZ involves minimal weight, a small space in my bag, and provides image quality and AF performance equal to the lens on a DSLR.
There are a few things tht are unique to mirrorless right now. The EVF is like a heads up display. I wear reading glasses and hated using LiveView because I had to put on glasses, then take them off to use the viewfinder. The EVF allows full menu access through the viewfinder. I've programmed function keys to provide zooming either to 50% or 100% through the viewfinder. This allows me to check focus or manually adjust focus before the shot, or check the image after the shot. I've used this function as electronic binoculars leading a bird walk and capturing record shots I could not get with a DSLR. The silent shutter is perfect for golf, and allows me to capture images during the golf swing that would otherwise get me ejected from the event. Focus peaking through the viewfinder allows precise manual lenses for some older lenses - some dating back to the 1970's.
There are lots of internet myths. The AF performance of the Z6 is equal to a Nikon D810 or D7200. Only the newest DSLR cameras with a separate AF processor have better AF performance for birds in flight or action. The Z6/7 has better and more precise focus for everything that does not involve flight or action. The difference is that AF funcitons that seem similar have changed. So some internet clickbaiter who has never learned to use the new Z's AF system, never read a manual, and using the wrong settings for the shot is claiming that AF does not work.
The Z cameras are similar, but have important differences. There is a steeper learning curve than any prior Nikon DSLR since the first models. I'm heading on a trip in a few days, and the Z6 is my second camera - paired with the D850. I could easily use the Z6/7 for almost everything I shoot without limitation for professional work.