Wouldn't worry me because environmental laws mean that, although it is legal, it must be in a position which doesn't cause a problem. No different from burying pets in a garden in my opinion.
However, the family doing it should bear in mind that if ever the property or garden gets a makeover, then they might feel dreadful about Mum turning in her grave (literally and metaphorically).
My mother buried her partners ashes in her garden and planted a rose over them. We dug up the rose and transplanted it when we sold her garden after her death for development. It was the least we could do for her and for him.