I havent had an issue with any of them mentioned so far. For me the only thing holding me back is bosses/mini bosses. Goddamn war den dropped me like 30 times before I beat him and as far as the Thresher/Overlook portion i just immediately asked for assistance. Game is so biased against solo players.
Currently Playing: Borderlands 2 : Maya Lvl 50 - Gaige Lvl 55 It's not until you lose everything that you are free to do anything