As some may know, I love Bethesda, but don't give them any slack when it comes to how buggy they make their games. I praise them when they do AWESOME, I come down on them HARD when they make buggy games, or even LIE and come down on them and will not let up.
That being said, Todd Howard has changed and didn't make any lies or promises as he did with Oblivion when it came to Skyrim. Now that we only had what an 8 month warning of the Fallout 4 being released and it took a long time to even acknowledge that Bethesda is making Fallout 4, the extra time for the game to be released, I am sure Bethesda is working really hard to make Fallout 4 less buggy on release. They always get a black eye (justifly so) for how buggy the games are on Day One. I think Todd and company don't want this so I would say this would be Bethesda LEAST buggy game on release.
Yes there will be bugs. Yes there will be game breaking bugs. It just will not be as bad as previous releases. I am only guessing and have no facts for this.
So the quality will be there. I will be SHOCKED if there is not. The game will be very playable at least on Xbox One/PS4. PC is a different story since all the combinations (different OS, different video cards, memory, chips, drivers etc) that for most will be playable but some will have issues that they can't play it.
If you are really concerned don't pre-order the game and wait a week to see if other people are having problems or not.