yes there are game breakers but i have had no problem. the game drops your quests and when you reload they come back. so you load game... quit to character select... then reload = no problems. more info here....
hub levels have many different people and quests. main levels like fornost have many parts.
Bree (hub level) Fornost Sarn Ford (hub) The Barrow Downs Rivendell (hub) The Ettenmoors Mount Gundabad Nordinbad (hub) Mirkwood Urgost's Lair Nordinbad Carn Dum
a playthrough is pretty maybe 8- 10 hours? but once you beat it you can go back and replay on heroic and then legendary difficulty keeping your stats and weapons. online is great because you can play higher diffs and get better loot.
sp and mp are great. mp still active.
the game has char levels up to 40 and skills. it may have in-game stats but i'm not sure. it's so good i beat it 3 times in a row and then deleted my save so i could start over from scratch.