I’ve only really played New Vegas extensively, but I think the biggest thing to remember is that when you’re familiar with these western-style RPGs, they seem like they have a bottomless well of content because you know how and where to find quests and such. When these games are new, you have to find content for the first time, and if you’re not sure where to look then it becomes frustrating and boring.
New Vegas is better at this than most fallout games because once you have a quest it often leads to a sequence of other quests. in FO3, half the game is just finding stuff to do. Never played 4 or 76. No one plays FO1 or FO2 anymore but they sometimes suffer from similar issues, especially FO1.
Build variety in these games comes more from the perks, skill pts, and SPECIAL stats you choose. They’re not quite as visual as the different classes in Skyrim, but they certainly do much different things.