I think you'll find that for it's time, the chestburster scene was about as shocking and gory (quite literally visceral) as things got outside of the really nasty exploitation movies...and presumably actual snuff films.
Sure, it wasn't wall-to-wall blood and guts, but that's the point. It's a suspense horror, not a splatterfest. If chests were exploding every five minutes then it lessens the impact.
Those two or three scenes are all you really need. Hell, I still put Lambert's death as the most shocking and visceral for what you don't see as much as what you do because of the time it took and, noises she was making and the state her body was in when Ripley finds it. Almost nothing they could have shot can compare to what your imagination can concoct as to what the Beast was doing to her.
As for Prometheus...I find it a very frustrating movie. On the one hand there are things I really like, such as the visuals, the concepts and the cast (for the most part.) But ultimately the substance of the story and plotting appears to have been sacrificed in favour of pacing and when you get down to it, that's a recipe for a poorly told story no matter which way you slice it.
