I replaced my 60D with an M6 II a year or two ago. Wanted something smaller that I am more likely to actually carry when hiking etc, but with enough MP for large metal prints. Overall been pretty happy with it.
However, with the EF adapter, and "large" lenses like 70-300L and (as of today) 100-400L II, the body/adapter do not seem structurally sound.
When picking up the camera with either of those lenses attached, I can see a hairline gap open at the top of the adapter/body interface. This can't be good for image focus, or sealing, or structural longevity... There also seems to be slight rotational slop in the adapter - can rotate it (or the lens in the adapter?) a fraction. It's always been like this.
Has anyone else noticed this?
NOTE - I am *not* complaining about the weight/balance - that's something I knew when I got the body, and am OK with.
You should never pick up a camera with large lens attached to it by the camera body. Your observation should make the reason for this clear. It creates too much [torque] on the mount. This is true of any camera body, and becomes more acute with the use of a mount adapter.
The EF-M mount adapter can make use of EF mount lens, but I would stick to the smaller, lighter lenses, which do not consume much power. The biggest lens I ever used on my M3 was the Rokinon 14mm T3.1 cinema lens. I tried the EF-S 55-250mm that I had, and immediately realized that this is not going to work very well.