A couple of things/problems to point out:
Issue # 1
When using the sale_discount module with the sale module, you see the discount information as part of the sale line. But if a discount is offered to a retail customer, there is no similar feature.
Issue # 2
When a product is placed on promotion using the sale_promotion module, the correct discount is applied when quoting a sales order. But when selling at PoS (using sale_point module), the promotion is not applied.