Late to the party, but how exactly does one screw up longsword? It's braindead (and incredibly forgiving) to chain, has no additional modes,does thoroughly decent damage. You don't ever need to learn when and where to block with it given if you're someplace where you can't dodge or backstep swing you're already screwed ... and if you're incl. flurry (which you should be doing everytime you get to 4/5 of a charged blade if only to recharge sword power and penetrating tough monster bodypoints rather than constantly resharpening) it's not even that slow.
Given it's range, its decent power, and its straightforwardness of chaining attacks it's also pretty good at targetting specific body parts for breaking.
Really good at breaking wings, head and tail....
About the only downside to the LS is the durability, and even then it's not that bad and in multi given the number of additional aggroable targets the team has to allow you a quick resharpening without going too far, it's not that big a deal.
LS is all around great, it's been my goto weapon for farming bosses since it became a fixture in the series.
That being said I prefer the switch axe for sheer coolness and the fact that I like having weapons where you actually have to think about your attacks and when and where to transform them.
I think the reason why th game recommends LS is not only is it forgiving to chain and to flurry, but it also forces you to learn when to dodge enemy attacks far sooner than any other weapon. I take a longsword into a fight with new creatures solely on the principle that by the end of the fight I'll probably be pretty decent at knowing exactly when and how to roll and use those invicibility frames, and exactly where to position myself.
I will say the Monster Hunter World feels like the easiest monster hunter to date....