Why don’t I like double and triple mounts? (e.g. triple would be something like a 400mm f/2.8G lens with 1.4x teleconverter on an FTZ adapter)
The simplest explanation is this: how are you going to carry that camera-FTZ-teleconverter-lens combo?
You absolutely don’t want to carry it by the camera. You’d be putting a lot of pressure on the mounts, risking a break. You’d be well above what Nikon specifies as the mount capacity of the cameras. The Z-mount on the camera is no different than the old F-mounts on the DSLRs: it’s designed to break in order to protect the rest of the camera. A few lenses have a break point built into them forward of the mount. The most notorious of those was Canon’s 70-200mm f/2.8, which came apart in the middle of a photography session for me, too. I’ve even seen a dropped triple mount situation where all the mounts had to be replaced. Not inexpensive.
Fixing a broken mount is a lot cheaper than fixing a bent frame (and Nikon doesn’t fix bent frames ;~). But you don’t want to be fixing mounts at all, if possible.
So you’ll be carrying the combo via the lens. Two adapters and a potentially heavy camera (Z9) hang off the back of the lens mount when you use a teleconverter with an F-mount lens. Same story: lens mounts are designed to break in order to protect the structural integrity of the lens, so beware of putting too much mass at the end you’re not carrying.
Still, the relevant point is that the camera and lens makers have designed their products to more easily break at the mount or in a fixed position just forward of the mount on the lens, because that’s cheaper and easier to fix.
Beyond that, as you walk about with a triple mount solution, the weakest points are the mounts, and they’re getting stressed as you do. All of you who claim you want metal mounts because metal is “better" should know that metal brasses (gets worn) when constantly rubbed. Enough brassing and the mount gets a teeny bit less tight. Keep at that, and you might find you need to perform an AF Fine Tune to keep the lens focusing correctly.
So any slop in the mounts doesn’t help, and if you’re constantly carrying a three-mount solution, I’ll bet that you’ll eventually have some.
One thing I’ve noticed over 20 years of teaching workshops with a lot of big, heavy lenses in tow, is that it takes less of a hit (or drop) on the lens to break the mount when you have multiple mounts in play. The more mass involved at both ends (lens and camera), the more likely a break occurs. Breaks may be rare, but they’re not extremely rare. You want to avoid them whenever possible.
There’s also the issue of extension. A teleconverter and FTZ adapter can add another five inches+ to the length of what’s sticking out front, which provides more opportunity to bang the front of the lens into something.
The FTZ Adapter instructions basically say to limit the mount load to just under three pounds (46 ounces, or 1300g) when not supporting the lens (e.g. when holding the entire load via the camera; I believe this is one reason why the original FTZ had a tripod mount on it, as you could support there and better balance loads on the mounts, though perhaps not with the big, heavy exotic lenses). Quite obviously we’ve seen Nikon themselves suggesting (and actually) using heavier exotic lenses on the FTZ than that, but that just gets us back to my first point: don’t carry the combo by the camera, carry it by the lens. Curiously, neither Nikon mount teleconverter manual mentions the same thing, but I’d assume that there would be a limit for them, too.
I like Nikon’s most recent solution (teleconverter built into native lenses). It reduces the mount slop and breakage issues (though you still have a heavy lens you should be supporting directly). It doesn’t hurt that the 400mm f/2.8 S is such a good lens, even with its internal teleconverter invoked.
Bottom line: if you’re using F-mount lenses—particularly with teleconverters—on your Z System camera, be careful about how you support, carry, and transport it. Support and carry from the lens, consider adding a plate that stabilizes the lens/camera connection, and always separate all the components for transport.
