There's going to be a lot of bugs. There should be a lot of bugs in a game advertised at this scope.
If you don't have bugs, you don't have real emergent behavior, and your "huge game" is shallow. Polish the gunplay, the stealth, the basic mechanics that people use every day absolutely, but you can't conceivably test every inch of a massive world and all of the varied quest lines with all of dozens of combinations of perks, plus weapons and followers people will play with. If we pretend there are 3 archetypes and 3 levels players could be at when entering a given area*, with 5 different base weapons and 5 different mods per weapon, you already have 225 combinations to test that quest at to find every edge case. And testing a quest for a specific combination isn't one play through. It's several different ways to approach every potential interaction. A character not being a stealth archetype doesn't mean that a real player won't try to approach an encounter with stealth. Also, enemy behavior isn't (or shouldn't be) linear and predetermined, so doing the exact same thing should result in different outcomes, and you have to allow for that, too.
You can't genuinely comprehensively test systems with the complexity a game of the scope Skyrim (and we're hoping Starfield) has. Even with an unlimited budget it's just not doable.
*obviously there are way more combinations of skills, but just for illustration.