DS are not as difficult as you might think, but they do require a lot of patience and caution to get through. one wrong step can potentially lose you a lot of progress. Thats were the frustration comes in. if youre the type to play games for relaxation or to rewind from a hard days work then these are not the games for you. People play them to explore its harsh world and best its challenges. If youre willing to get through it they could be the some of the best experiences youll have this generation, but there not going to be for everyone.
DD is probably more your taste. much more adventure focused. it can be difficult at the beginning, but as you learn how things work and upgrade your self its difficulty drops dramatically.
as for BL1 and 2 they are pure shooters with some slight rpg elements. 2's singleplayer fares better than 1's thanks to more diverse environments, enemies and characters to interact with. however they can come off as feeling mmo-ish with their quest design making the singleplayer substantially less fun than the coop (assuming you play with friends). They do have a LOT of content though and as long as you like the idea if shooting lots and lots of things and getting lots of lots loot for dozens of hours then their good choices.