I recently got the MMF-3 and, having already had a Viltrox JY-43F for some time with less than spectacular results, I thought I'd compare the two parts.
The biggest problem I had with the Viltrox was corner sharpness and in particular with wideangle lenses, where the corners were, frankly, appalling.
I measured the MMF-3 to be a consistent 19.2mm (front flange to rear flange) all round (as you'd expect). The Viltrox I measured to be 18.7 to 18.8mm. In other words, the Viltrox is both undersize and positions the lens at a slight angle. The undersizng is probably not so important because the lens will simply focus where it needs to and undersizing will guarantee focus to infinity. That said, half a millimeter undersize is a bit dramatic...
So, despite the Viltrox being just about twice the weight of the MMF-3, I would suggest that in the long run any of the OEM adapters will be better vslue than the 3rd party devices. You might get a good alternative but equally you might not, and that's what you pay the extra for.
For clarification, the flanges on the MMF-3 are metal as far as I can tell (some people think they are painted plastic) and the core seems to be some kind of plastic (a foam of some type, maybe carbon fibre) which is very light but does seem plenty strong enough even for my 50-200mm. The outer shell is plastic and the release button is much nicer in operation than the Viltrox, which feels a bit spongy. Without caps, the MMF-3 weighs just 42g whereas the Viltrox JY-43-F weighs exactly double at 84g.