That depends on which reviews you're looking at. Some of them are unreasonable, but there is a lot wrong with the game. Really. A lot. Starting from the basic idea and including most of how everything is implemented. It's buggy as well.
If you're into Fallout and you can ignore all the bad points of FO76 (there are a lot of them) and you enjoy exploring and reading notes and environmental storytelling then FO76 is mediocre. That's about as good a review as you'll see.
If you want a survival game, there are better ones.
If you want an MP game, either PvP or co-op PvE, there are better ones.
If you want a SP game, there are better ones.
If you want a Fallout game, there are better ones.
If you want those things badly mixed together, then FO76 might be your thing.
The controls on PC are startlingly bad, by the way. My guess is that nobody was assigned any time to work on them - it was just a quick and dirty port from Xbox.
I'd give it 4/10. Not as crap as some reviews suggest, but that's my opinion. Even Oxhorn, who makes a living primarily from Bethesda games, isn't recommending it. He's careful and polite about not recommending it, but he's not recommending it. And neither am I. I think it's worth a small amount of money to me, but I wouldn't recommend it to anyone else. I'm playing it. I'll probably play it for a while.
I can definitely see the play value (i have much better things to spend money on frankly), but the longevity is clearly not there going by high-level players and basic gameplay loops being rather meh. But it does just feel like a game of instant gratification chasing, which pretty much began when they ripped out the loot system of Skyrim and made FO4's, which although is nice in it's own way, they should really find a combination that isn't so MMO like.