There's a bit of difference though, it's how they're used.
The Ark. We know the biblical stories, we know the history of Tanis. But there's always a moment of 'what the hell are we messing with' in it. The Ark is never explained in detail, it's never really 'used'.. it's a mysterious entity all its own that WANTS to stay hidden. And it is... buried for 22 years, out of the hands of the Nazis.
Sankara Stone. We actually don't even know what they DO, aside from glow and get hot. They're objects of some power and mystery, and, in the end, Indiana gives the 'item of luck' back to the people who rightfully owned it. We're not given, however, a 90 minute lecture on the history of Shiva, etc... though we do get just enough explanation to know what it is.
The Grail. While a bit more explained, it's in keeping with what's come before. We're told the Grail lore in backstory, and then it's off to the quest.
Akator. Suddenly, things change with the aliens. Every conspiracy theory from the Cold War (such as the man-I-never-want-to-hear-about-it-again Roswell) is back with a vengence. Do we hear about their influence, is there any mystery or sense of awe about it? About as much as seeing Greedo shoot first.
And, to drive the point home, we even get the UFO shot, the live alien, and a bizzare psychic thing going on...
There's plenty you can do with aliens, and, indeed, Spielberg HAS done a better job twice in the past to be sure. But, this isn't it.
