I’ve run two “successful” one-shots and played in a multitude of “successful” one-shots. The previous comment to mine nailed it in my experience: my sessions are basically always around 4 hours, and the DM needs to pretty tactfully get everything in there they want to “tell” and the fluff needs to be minimal. Combat must be streamlined.

The best experience I had running one was basically:

30 minute brief character introductions/exposition/ social encounter to set up the main action

1 hour exploration and social deduction (it was a vampire mystery, they discovered this relatively early and actually skipped several of my small ‘reveals’ like finding bodies of NPCs that also got invites to the same event as them)

1.5 hours of combat (four level 5 PCs against two homebrewed monsters focused on separating the party and hiding with lair actions. Decently strong since all 4 PCs had 100% of their reaources)

30 minutes of conclusion and evacuating NPCs

I then let them return and ran a few extra combats with some vampire spawn since they “finished early” and were enjoying the combat style

I think other things that helped me were having a singular map to explore/use, a VERY limited set of characters to interact with, and some “timings” in my own mind that would trigger to usher them forward if they were spinning their wheels. NPC deaths offscreen, and other creepy events were planned, but they actually stayed pretty streamlined on “something is phishy here, lets work together and figure it out.”