The Scroll of Secrets is worse than the one, that's for sure. But Mystics probably haven't access to that, so let's make a few points on Scroll of Secrets in a pure after playing it. Yes, most things will work with the counterpart. I'll speak exclusively about the Tabooed versions (with the ability).
- If you go Down the Rabbit Hole (maybe you primary upgrade your spells just with Arcane Research) this costs you 2 XP, which feels like the right spot experience-wise.
- 4 Secrets for 1 Resource is very solid, especially if you want to spread secrets with Eldritch Sophist across the table. Only Encyclopedia (2), Schoffner's Catalogue and the upgraded Ancient Stones may offer the same ratio, while Forbidden Tome (0) and Forbidden Knowledge are slightly above.
- If you use all the secrets for draw (not discarding, not leaving them there and not for manipulating the encounter deck) overall you get 4 extra cards in windows (one more card than the Scroll, though you can't pick one of three).
- There is a bit more competition for hand slots these days, but in general Mystics have hand slots to spare, unlike Seekers.
- Gone empty, it's a great target for Dexter Drakes ability or for Sacrificing to get resources or even more cards. If you have access to level 1 cards obviously there is Library Docent, too, for repeating the fun.
As people mentioned, the most interesting difference to the Scroll is looking at the top card of your deck and being able to discard weaknesses you would have drawn next. With the one you can delay them (putting cards from the bottom on top), but probably you have no knowledge of weaknesses sleeping at the top of your deck.
The effect sounds like the old Alyssa Graham Trick protecting you from Doomed and stuff, but therefore you just put the card at the bottom of the deck (drawing it later) and have to add doom. Also if you're like me you could forget, that you shouldn't activate effects which reshuffles your deck...
That said, Mystics are the masters of “look at” and “return in any order”, so you have a chance to manipulate your deck in order to discard nasty weaknesses like Doomed, Offer You Cannot Refuse or Unbound Beast. In the past there wasn't much more than Scrying, but with the release of EotE we have Parallel Fates (2) to celebrate, which lets you look at 6 (!) cards. In mid-/lategame (or using Underworld Support for smaller deck size) there is a realistic chance to draw an important card with Parallel Fates (2) AND discard 1-2 weaknesses with the Scroll. And even without manipulating effects, I randomly hit a weakness or two with the Scroll and discarded them in my play-through (which may happen often in the Edge of the Earth campaign due to spoiler reasons).
The option to look at the bottom of the deck is neat too, if you've drawn Doomed and want to discard Accursed Fate or if you have Wendy's Amulet or a Copycat player at the table. Speaking of this, as with the other Scroll of Secrets it's a great multiplayer tool because you don't have to be at the same location in order to look at another investigators deck.