It is okay if the opponent has action-VP cards in play. Because those go back on the deck just fine, you can still reduce opponents to a 0 card hand with B pins.
The biggies are:
1. A lack of useful draw for your opponent. Even something terrible, like KC/pearl diver can go a long ways toward making the pin impossible to hit. Whenever the opponent gets to play some draw, they get the green off the top of the deck, out of their next hand, and into their discard. Some drawers, like shanty town or apprentice, just aren't useful enough for the other guy to get them so you might be able to work something out with them because other players won't touch them. The only big exception to this are dual action-vp cards that you can force out of their hand. Nobles and great hall are dead if they can never stay in hand or be drawn.
2. A way to deal with silver. Watchtower is the obvious one, but it is draw for the other player. Another option is apprentice - a good drawer for you, a bad one for someone without a steady flow of cards they can crunch. Afterall, once you start pinning, they will have crappy hands and bad odds of matching their apprentice up with something better than a copper.
3. A way to quickly get a lot of Bs in play. Let's face it if you try this pin without KC or TR, it's going to be very painful to get all the cards you need/want. 5 Bs, 4 villages, at least one drawer, and a watchtower is going to 11 turns before you can start pinning - and somewhere in there you needed to trash down to make that remotely viable. You might have better luck with IW, but even there you are looking at hitting this very late. Because TR and KC can both chain cards as psuedo villages and allow for multiple plays of a single card they drastically cut needed card totals.
4. It helps if you have something to put some green on top of the deck. Rabble is good, but it draws so it can be a very mixed blessing. KC/Rabble is a good shot at avoiding a B pin. Fortune teller is actually better than Rabble, IMO. This means that once you go for the pin, you can always fish another green out of their discard and put it onto their top deck.
5. Cutpurse. Cutpurse, like B, is unlimited discard. It can be very useful to have cutpurse discard their coppers so that like some conditional drawers like spice merchants, stables, and cellar don't work. Likewise, it can be really useful at keeping your opponent below the 5, 6, and 7 coin thresholds so you can build the lock before they can get their own KC combo going.
6. Ghost ship. Unlike all the other discards, Ghost ship forces a top deck so they cannot just discard the VP cards before you play B; however it is +2 cards which can make pinning that much harder.
7. Useful aternate VP. In order to have any chance at a B pin, you need there to be reason for a good proportion of the opponent's cards to be VP. If you try to B pin when the opponent is hitting colonies, it is going to be painful. If you try it when they are massing silk roads or even nobles, it is much more likely.
All that being said, I've only ever gotten this to work with apprentice for KC/apprentice/B/chap and watchtower for KC/KC/B/B/Watchtower (with island) against any close to compotent opponents. In both cases, it was with selected sets for the purpose of testing this. In both cases I could have won sooner with other better strategies.
