The major problem of Fallout 3 is that New Vegas came out afterwards.
It was GOTY when it came out and it was absolutely fantastic. Sure there were flaws and the game has aged but it also came out nearly 10 years ago. A lot of the whining about Fallout 3 is revisionism from people who played hundreds of hours in the first place.
I don't actually hate Fallout 3. But at the time when it came out I really had this feeling that the game could've been better. And that it was something mods wouldn't be able to fix.
Then New Vegas came out and fixed everything
NV still pissed me off with how railroady it was in terms of where you could go. You had basically one path you could feasibly take until a good ten levels in or so. Fallout 3 had a helluva lot more - sure, plot was shitty, but I've finished a grand total of one Bethesda rpg out of all the ones I've played. And that was Fallout 4.
This isn't correct.
In Fallout New Vegas you can go anywhere you want, you can even proceed north up to New Vegas right from the start and skip the entire first 3rd of the game. Almost all speedruns do this. It's just very challenging to do because it's infested with Cazadores and Deathclaws And even when you get to New Vegas there are several ways you can enter the town, and several ways you can approach obtaining the Platinum chip you need to further the story. By comparison in Fallout 3 it give you the illusion that there's variety to the story but in actuality you're doing the same quests in the same way each time and usually a speech check just lets you bypass having to shoot dudes.
Not to mention the entire second half of the game is entirely open ended, you're given the option to choose which faction to side with, which drastically changes what quests you go on (compare a Legion playthrough with an NCR playthrough and several quests are almost reversed in design, the entire climax is different etc), and most of the quests can be tackled in any order and take you all across the map.
In Fallout 3 you could go conceivably anywhere but this didn't lead to quality game design. Most rpgs are designed in such a way that challenging areas are open to you at the start so you can go back and lay waste to them when you're stronger. I linked a video earlier in this thread where someone specifically points out at the start in STALKER you can optionally assault a military base close to the start of the game and it's intensely challenging but intensely satisfying to do.
In Fallout 3 there's very few instances of this occurring, where the content actually ramps up and you go "I should come back later when I'm stronger". I can only think of a handful of moments and right now the only one that comes to mind is Old Olney. It's why there is an extremely popular mod called Fallout 3 Wanderers edition that is specifically designed to balance the game in such a way that the second half of the game is actually challenging where you don't feel like the terminator.
It's why it's not surprising the majority of players when they play Fallout 3 and especially when they replay it, they completely ignore the story and instead do the more entertaining quests or just wander around the world like it's Grand Theft Auto. This isn't the case with New Vegas where the story is actually good and the quests are varied enough that it's worth replaying the story again.
The approach New Vegas took to game design was intentionally designed to pay homage to the original Fallout and Fallout 2. Wheras Fallout 3 was more akin to Oblivion. Which had a similar problem where the storyline and characters were garbage and the majority of players ignore it.