I think that Return To The Dunwich Legacy is unique in that having it in your collection might make your deckbuilding more difficult as opposed to easier.
The reason I say this is because of Shrewd Analysis. If you're looking to upgrade into 2 copies of Strange Solution: Acidic Ichor, using Shrewd Analysis, then the presence of Strange Solution: Empowering Elixir is going to make the odds of that happening a fair bit worse. So, if you're looking to use Shrewd Analysis, you're better off using it if you don't own Return To The Dunwich Legacy.
Additionally, Return To The Dunwich Legacy also adds a new basic weakness, Through the Gates. Normally new weaknesses are included in deluxe expansions, and in the deluxe expansions the inclusion of new weaknesses is offset by new investigator options and more level 0 cards. No such luck here, though.
As for Strange Solution: Empowering Elixir on its own merits - I don't think it's the most useful version of Strange Solution out there. It's one action to draw a card and get 2 resources. You can do the action 3 times. It costs you one action, one card, and one resource to get it out into play. Of course, that's also not taking into account the other, less obvious costs: having to "identify the solution" on the level 0 Strange Solution (which costs one card, one resource, and at least 2 actions earlier in the campaign) and then 4 experience to upgrade into it. That sort of cost is fine with something like Strange Solution: Acidic Ichor because you're getting a really powerful weapon in a class that doesn't normally have access to firepower like that. By comparison, Strange Solution: Empowering Elixir is underwhelming.
An interesting card to compare Strange Solution: Empowering Elixir to is Stand Together. For one action, Stand Together nets your team +4 cards and +4 resources. It costs 0 resources to play and 3 experience to include in a deck. Strange Solution: Empowering Elixir gets you 6 resources and 3 cards, but it costs you 4 actions (the 3 to trigger the ability plus the one action to play it) and one resource to play the card. It costs one experience more than Stand Together to add to the deck, and you need to spend all the extra energy to to "identify the solution" to even use it in the first place! Maybe it's better to leave this kind of card/resource support to Stand Together Guardian investigators.
Unless support cards come along to make this card better, I think its primary purpose will be to make Shrewd Analysis a little more difficult to use than normal.