I would like to add a caveat to the matrix style of assessment.

The methodology fails with high consequence hazards in low likelihood occurrence areas

If a risk is seen as life threatening it should always be graded as high (16 in the example above) if it can happen, whatever the chance of occurrence.

I cannot countenance risk assessing something that can kill without putting into place any and all accommodations that will reduce that risk level.

Life threatening scenarios require very expert assessment by competent people.

(Here starts the discussion of 'competent' on this forum)

In the UK one definition often used is:

"To be competent an organisation or individual must have:
Sufficient knowledge of the tasks to be undertaken and the risks involved.
The experience and ability to carry out their duties in relation to the project, to recognise their limitations and take appropriate action to prevent harm to those carrying out construction work, or those affected by the work".