This box was found when a wall was opened up. It was apparently closed up in the wall during a Kitchen renovation that was done.

What are the issues here? Why is this not allowed?

For starters, there is no access to check on the splices if there is a problem. There are 2 cables in a clamp that is designed for 1 cable. The cables are not fastened to the framing within 12" of the box. Hard to say from the photo, but it looks like the black cable is of a smaller gauge than the white cables and hopefully the black cable is NM.

This looks rather similar (but not quite)
to a job, I was doing down in the lower South Island of New Zealand, after I came out of my time, some years ago.
This job was the renovation of a Police Station, the builders took the old Fiberous Plaster linings off of the walls in the Seargents Office and on one wall, there was
a joint in there that would have been beyond the belief of any electrical person in the world.
There were a set of three brass busbars, nailed to the stud of the wall, obviously for Phase,Neutral and Earth, and from this most of the socket-outlets were fed, there were wires every where, most of them conduit wires, I wish I had have had a camera, back then.