As it stands, melee gives the same amount of toughness independent of the difficulty of killing said enemy.
Hordes are relatively non-threatening because they are toughness farms which you can use to legitimately ignore being shot because they can let you generate faster than you lose.
Despite shooters draining a ton of toughness, you are not well-rewarded for taking on shooter groups and managing to close the distance. You are punished for not managing to herd each group of shooters together. If you are actively fighting one, another can be shooting you, and you’re much more likely to lose toughness since dodge sliding is much more limited than sprint slides. This weakens melee gameplay since it makes the right play to play range and stay safe and not giving some sort of sustain to keep going in melee.
Specials are generally more threatening and harder to get to in melee, and yet they offer the same amount as a random poxwalker.
I personally think the melee side of Darktide can benefit a lot from differentiating restored melee amounts scaling with the threat of an enemy. This feature was already a thing in Vermintide 2 with THP on Kill giving variable amounts based on the enemy.