I’ve used a Kenai holster for carrying various handguns around the farm, including a G40 MOS, G21, Walther PPQ M2 9mm 5”, and a 6” 686. I most often carry the G40. The holster shell for the Walther is too tight, imho, as I have to really work at it to get the gun out of the holster. The others have good retention, and release cleanly when drawing.
I have no experience with the Diamond D, but it looks like a fine rig. My closest experience to it would be a Galco shoulder holster (vertical carry) which I bought a couple of months back for my 1911; I’ve also been carrying it on hikes around the farm to get used to it. It is very comfortable, and I look forward to wearing it out and about as the weather gets cooler. I bring this up not as a substitute for the chest holster, but because it’s leather, and it gets wet from sweat. I could see where it could possibly stain a shirt worn under it.
The Kenai, being made from nylon webbing and kydex, doesn’t seem to have this issue.