New weapons are across the board in terms of usefulness. Actually lately they've been very interesting and unique, and difficult to place in a tier list because of the risk/reward properties DE has put in. Not sure what you mean by special encounters. The only resource that isn't immediately available during a mission is Nitian Extract, which comes in the form of an often occurring (3-4 a day) alert mission.
I literally could never get vauban even though I thought he looked the coolest because his parts came from waiting until some super secret shadow ninja assassin dude appeared and I would have to kill him, kill each of those trio ginyu force guys and some other special random encounter. It felt like I maxxed out the game and everything after that was better to purchase to save time and headaches, even though I only cleared half the solar system(no one plays past a certain planet, everyones on the last 1-2 planets and the dark zone or whatever it was they added. Term was taxi right? Nonexistant).