Weak was not the right word. I guess I mean often not very useful. Aside from synergy with the cards you listed, I am struggling to think of scenarios where I would want this. Maybe if I have a Silver gainer and there was a $2 on the board I really wanted (a lot of)? Or Gold gainers if I need $5 Action cards.
I totally get your point that “costing up to $1 more” would be broken if for no other reason than Loots become Provinces, but maybe a good compromise would be “…may exchange it for a card of the same cost…”
I agree that it does not work in the absence of some synergy with another card/landscape; but that is true of a lot of landscapes. The most obvious example of this is Tomb which, in a game with no trashers, cannot be used. Similarly, most action cards aren't worth playing when they are gained, making Innovation unbuyable on many boards without either a gainer or some matchup.
I also think there are a ton of instances where this would be useful. There are more than 25 cards/landscapes that gain Gold specifically.
1 In a number of cases (Bandit, Soothsayer, Swashbuckler, Tunnel [with discarding], Market Square [with trashing], and Acolyte) a strong engine would be able to trigger the ability frequently, possibly even every turn. But dumping a bunch of Golds into your deck is not conducive to having (and keeping) a strong engine. But trigging it every turn and instead taking a $5 (or even $4) engine piece could be extremely beneficial to building a strong engine. At the end of the game, a Gold can be exchanged for a Duchy. And all of this works with Loot gainers as well.
There's strong synergy with type-specific (or cost-specific) on-gain bonuses--Ironworks, Replace, Academy, Guildhall, Groom, Livery, Galleria, Band of Nomads, Taskmaster--and allowing you to take their respective bonuses without (ultimately) getting a card that matches.
Similarly, gaining effects that put limitations (other than "costing up to") on what cards you gain can be assisted by Alchemy, as you can exchange an available card for one you want more. You can do this with Treasure gainers (Mine, Pirate, Tragic Hero, Hero, Silver Mine), cards that gain from the trash (Graverobber, Lurker, Rogue) or those with other restrictions on gaining (War Chest, Changeling, Gather). Until the pile empties, when a Tragic Hero's power triggers, you can gain a Gold, then exchange it for another TH.
There are other benefits as well. A Border Village can turn into 2 Duchies. A Port turns into a village + Silver. A Blessed Village turns into a Boon + Silver. A Skulk turns into a Gold + Silver. Cemetery turns into trashing + Silver.
Thus, while Alchemy does require some kind of a synergy to be worth buying, there are a lot of them out there, some of which are already quite powerful/effective.
1Bandit, Courtier, Transmute, Hoard, Soothsayer, Demesne, Fool's Gold, Tunnel, Market Square, Treasure Trove, Quest, Fortune, Haunted Castle, Wedding, Devil's Workshop, Leprechaun, Skulk, The Sky's Gift (Boon), Swashbuckler/Treasure Chest, Commerce, Enclave, Reap, Acolyte, Territory, Doubloons, Gold Mine, Governor.