the other issue with lunging is that your ROM is limited and or restricted by the opposite leg, meaning that you are only doing a sort of half squat which is not ideal and would need to supplement other exercises to improve hip flexibility (glutes), you need to go much deeper, by using the hammer piece, you can place the opposite leg on the bar behind, and perform a full squat position safely, much more safely than the barbell version of the "full" squat which is impossible for most.
If one can do a full lunge using heavy weight safely, I feel that there is no need for any other lower body exercise
From my experience I've noted that the first muscle group to often suffer over the years are the glutes, as these big muscles rarely receive a good workout over the years, and most older trainees blame weak legs when in fact it's the glutes that lose strength and flexibilty.