generic h40 needle scum build
 Hive Scum Build - Warhammer 40k: Darktide 

the three most replaceable nodes are In Your Face (used for needle, significant dps increase), Hyper-Violence (used for cleaving trash, which might not be necessary on other melee weapons), and either Swift Endurance or Float Like a Butterfly (both huge dps increase for melee, not very significant for needle or weapons with low finesse modifiers)

Fervor  can be replaced and mixed around to whatever you feel like, I just love movement speed, and attack speed is a rare buff that effects damage, stagger, and toughness regeneration since you can trigger them all more often. I do not recommend the toughness tree, though, it is quite mediocre because of its poor uptime. I would say Kalma IV and attack Spur IV are the most valuable with strength being good if you need rending from Fury II , or if you want to take away attack speed in favor of strength on say, shivs.

"Why no carapace/unyielding/flak damage on needle?" The damage perks only apply to the dot when you swap to the weapon after applying it. Which means in order to get value, you need to shoot > swap to melee > swap to needle gun on every target, which is just too much APM for so little value. Just run reload speed, it's better on the mental load. Other perk is irrelevant.

Melee is for trash clear, which the needle struggles to take care of in the long run. It has great burst damage on groups, but it doesn't have the ammo economy without constant supplies of elites to make it a reliable trash clearer. Catachan Mk VII "Devil's Claw" Sword is chosen for its exceptional horde clear (H1 > L2 repeating) with little investment. It is also very good at dispatching elites with the push attack > L3 combo. It is not the best but it is very good at those two functions while also keeping a high mobility stat, which is one of hive scum's strongest traits.

Grenade can be swapped out for Boom Bringer if you play in Auric or Damnation where crowds are smaller and armored enemies need to go immediately. Aura can also be swapped to any of them depending on what you feel like, Gunslinger Improved is just for giving more ammo to teammates, it is not valuable to oneself with Pickpocket . 

I would say the biggest flaw with this build is a lack of stagger or other CC like Blackout  or Forge's Bellow to get oneself out of a corner. It has no way to immediately disable a group of enemies or a single enemy bigger than a mauler. But that is acceptable in premade teams or Aurics where you can go with rockets.