I was playing mythos draw like investigator phase, letting the team choose draw order differently each time. But in reality you must abide to the In Player Order rule, starting from the lead investigator then going clockwise always. Therefore an ability that reveal just 1 top card like this basically means seeing what's destined for the lead investigator. (What I was playing is supposed to be Gloria's special ability.)
At the same time to play with this card it is important to realize that the lead investigator could be changed each scenario, not fixed for an entire campaign. (my another wrong impression) It may matter when replaying a scenario when you know the density bias of encounter set, so you can choose the right person that could see the future through Alyssa. Or you can make the person with the most risk (e.g. full of trauma) to be the lead and continue protecting that investigator with Alyssa.
Just knowing the treachery is powerful as you can exploit its condition. For example something along the line of "attach to your location, nearest location with.." : You reposition and force it to have favorable choice you want, "lose resources" : You spend all the resources ahead of time.
Adding doom is expensive, but generally imo just having never out of charge way of looking at it is enough to improve the chance of dealing with the encounter massively (e.g. just group up to prepare commit icons), as opposed to Scrying and "miss" because you don't see anything interesting. Doom bury is just an extra icing to cheat a certain defeat that you see revealed, like an Elite enemy coming up when no one is equipped to fight.
Lastly doom is placed on Alyssa and not the agenda. It is not hard to put 1-2 horrors on her early so she can die swiftly with the doom if you are forced to use it.