I looked at the Oxycheq soft plate pretty critically and to me it works, but it doesn't work that well.
Here are the problems:
The only 'rigid' part are where the cam buckles attach the plate to the tank and the section in between the cam bands. That is the only part that's truly rigid. The section between the top cam band and where the shoulder straps exit the plate, that section is flexible and prone to twisting and moving.
The sections in between from where the lower cam band attaches to the outer harness attachment points is also flexible, and the flexing happens from the tank all the way to the lower outer slots allowing the tank to roll side to side even with the harness cinched tight.
However, there are limits to the maximum amount of movement until the system corrects itself, but there is a wide area of allowable movement. If a diver is OK with this movement and understands that these soft plates are like this then fine.
But they're not going to be as stable as a completely rigid plate which holds all the harness attachment points in complete rigid alliance with the tank in it's entirety.
I've experimented with doing away with the plate completely and just using the tank alone as a rigid component (the tank is the plate), and even this works better that a soft plate since the harness attachment points go directly to a hard point (the tank). The limiting factor is the wing in this situation; most wings don't have slots wide enough apart (side to side and top to bottom) to make this idea work well. It requires modifying the spacing of the slots to where the cam bands going through the wings slots is in an ideal position to hold the shoulder and waist straps in the proper position.
I'll try and post a pic of what I'm talking about for those who are interested.