Fallout provided neat perks/traits that substantially-impacted how one played; that's a signature part of the series. The great bulk of the perks in The Outer Worlds were things like small percentage increases. They didn't have a significant impact on how the game played out.
The weapons didn't "feel" very different other than across classes, with the exception of the "science weapons", so there wasn't a lot of variety in gunplay over the course of a game.
While the world was open in that one could technically always backtrack, there wasn't much reason to do so.
Most of the content was in "cities". Yeah, sure, there was wilderness, and maybe that added a sense of scale, but it was mostly just filler between cities. If you're wandering around in Fallout: New Vegas or Fallout 4, there was interesting content all over to just stumble into. One only really got that in cities.
Not a lot by way of meaningful, world-affecting decisions. Okay, you can also criticize Fallout 4 on these grounds, but if you were hoping for a Fallout: New Vegas...

There were some things that I did like. In particular:

It was pretty stable and bug-free. The Fallout series has had entrants from a number of teams, but one consistent element has been a lot of bugs at release.