Younger fan here (turning 15 at the end of October). I wouldn't say I found either Prometheus or Covenant scary, but honestly, I didn't need them to be. As long as I could be invested in the characters, that's all I need. A good recent example would be It. I wasn't really scared (with one notable exception, but that'd be getting into spoiler territory), but I didn't mind, because I loved the characters and wanted to see them succeed. I felt Prometheus also succeeded at that, on top of being visually stunning and having some great ideas on where to take the series. With Covenant, however, the only new characters I liked were Daniels, Walter and Tennessee. That was one of several problems I had with Covenant. And it's not like I went in hating it. Tied with Dunkirk, it was my most anticipated film of the summer. I wanted it to be good. I even saw it twice in the theaters. Alas, I just found it disappointing. Not terrible, not even bad I'd say, just meh.
Then again, I unironically like Suicide Squad, so maybe my tastes in film shouldn't be trusted.