The MD-B12 grips I have have a shockingly poor physical connection to the camera body. With the grip fully tightened against the body, you can grab the D800 body with one hand and grab the grip with the other hand, bend and twist, and actually see the flexing between the body and grip.
Hmm. The MB-D12 (for the D800/D800E/D810/D810A) is one of the better grips in terms of the rigidity of the joint with the body. With some of the other grip + body combinations, there is a lot more flex, e.g. with the D7100 + MB-D15.
I mainly find it an issue if I'm doing vertical close-ups with a short tele macro, and happen to have the grip attached with the D850 (MB-D16), there is some flex that makes it harder to get the composition right. So what I do is I simply take the grip off and mount the camera to the tripod with no vertical grip.
When using longer telephoto lenses, there is usually a tripod collar on the lens which means the camera is not mounted to the tripod, so the flex is not a major concern, mainly a slight annoyance.
Are the other camera manufacturers' accessory vertical grips better made in this respect?