I'm comparing this to Royal Carriage.
Besides technical ways in which they TR (RC can't TR self trashers, you can use multiple RC on a single action card) here are the important differences that I see:
Pros of Cabal
1) Can TR night cards- The caveat is that there aren't many night cards (15), and some of them aren't good TR targets (Guardian, Raider).
2) Can TR itself- This is very helpful if you are low on actions
Cons of Cabal
1) Can only save 1 for future turns (unlike RC which has no limit on how many can be on your tavern)
2) When you save it for future turns, it takes up a space in your next hand- You only draw 4 new cards (besides Cabal), whereas with RC you draw 5 new cards
Absent of its ability to TR night cards, Cabal is a TR that can save itself in a weaker way than RC. It would obviously be TR<Cabal<RC. It would have to be more than $4 (because it's better than TR) but it couldn't be $5 (because it's weaker than RC). The Q is whether the ability to TR night cards pushes it up to $5. It doesn't really seem like it unless there's a night heavy kingdom.
Good Analysis. A few points:
1. One other pro to Cabal is that it can't be drawn dead. If you Smithy RC, you'll have to wait until the next shuffle to (possibly) play it.
2. I get that one RC is likely stronger than Cabal. But I think there's room for both. Some kingdoms will have one, some the other, and it's only when they are in the same Kingdom that it's a potential issue.
1. You're absolutely right. I actually thought of that before writing my post and then forgot to include it.
2. Fair point (especially when adding in the pro of never being drawn dead).
One other point I want to make. I don't even think the Kingdom needs to be Night intensive. With just one Night card, the Night TR ability could be enough to make it worthwhile.
Cabal originally began as a cheaper TR just for Night cards, until I said "DUH" and realized that was useless on a Kingdom with just 1 Night card (you could just buy a 2nd copy of the Night card, since they are non terminal).
However, now Cabal offers you the ability to effectively get that 2nd Night card but transform into a TR when you don't want that.
For example, rather than get Two Vampires, you can get Vampire and Cabal. And once it's a bat, you've got options: if you have more than 3 cards to trash, you can still double bat with Cabal, but now, you don't have to. If you only have 1 or 2 cards to trash, save Cabal for your next turn.
Or Cabal and Changeling (use it mostly as a regular TR, until you have a desire to double Changeling (and you get to keep the Cabal after).
Even with Raider as the only night, using it occasionally as a +$6 next turn seems pretty powerful.