Why does Red Nose not recommend the use of sleep positioner if the safest position for a baby is to sleep on their back? Other than the risk of loose fabric covering the babies face or head or restricting movement of the head are there any other risks?

It is important to always place baby on their back to sleep
Once baby is able to roll both ways, & he is in a safe environment, place him on his back for sleep & allow him to find his position of comfort…......... that often is the tummy position.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

Products that restrict the movement of a baby or a baby’s head should not be used. This is because there are no Australian Standards for these products and case studies have shown that these products can be unsafe.
https://rednose.org.au/article/is-it-safe-to-use-products-designed-to-keep-baby-in-a-particular-position

Some of these products are made with soft, compressible materials, which might increase the risk of suffocation.
Some babies have been found “stuck” (entrapped) in unsafe positions, while in positional/restriction devices.
This can be particularly risky if head/face is covered.

Because of the lack of evidence that they are effective against SIDS, suffocation, or gastroesophageal reflux and because of the potential for suffocation and entrapment risk, sleep positioners in general are not recommended & many safety organizations warn against the use of these products.

Babies learning to roll is a normal development stage, & like all stages, the sleep environment should be made as safe as is possible….. which includes placing baby on back for sleep, move from bassinet to cot, cease swaddling & ensure cot mattress is firm, flat & well-fitting.