There are several general types of +2 cards engines:
1. Village (or draw, though I don't think we have any of these yet) has +cash. These are essentially cantrip in nature. Festival -> Moat is the same as a Grand Market. Making an engine out of it is, in theory, no worse than trashing your deck and massing Gm (or say Peddlers), if you have the means to set this up quick, make it reliable, or both it can actually beat racing to Gm (it is, however, a rare setup where swapping to Gm isn't more reliable later). Things like Scheme or Haggler can make this viable.
2. Village has +cards. This makes a +2 cards setup to be a lab. As with above, it is less reliable than just buying labs and unlike above, Lab isn't a power card. You need a really good reason for limited live draw. Something like Fortress/Forge/Courtyard can work really well to setup a quick Forge run.
3. Village or draw has discard. Take something like Inn/Moat. In this case you will never increase your hand size, but because you can use Inn to sift out the bad cards it does (in theory) let you eventually get to a power 5 card hand (e.g. Plat, Plat, Plat, Plat, Squire). There are actually a decent number of setups where this sort of thing works; you can also have the discard on the draw (e.g. Young Witch without the ability to give curses). This plays like a better, though again less reliable, version of spy. Works best when you have one or two painfully powerful cards to hit.
4. Edge cases. If you do just spin your wheels it can work for a few edge cases, mostly from cornucopia. Horn of Plenty is happy with more cards in play, Fairgrounds works better with even crappy wheel spinning engines that allow it to bulk point count, and Harvest likes more diverse junk in the trash ... these are extremely rare to be viable.
So, as noted throughout the big problem is that most of these setups really are just less reliable, more expensive (in terms of cash and more importantly often - time) ways to do something some other card already does. Given that Dominion is pretty good at appropriately pricing its cards, it isn't too often that you'd prefer paying more than list price on massed engine components. Sometimes, the or the village does something else, making it more viable (e.g. Festival/Oracle gets +buys, sifts, and gains cash) than a simple alternative (e.g. Oracle BM). Other times, you can more quickly setup a weird +2 card engine than power out BM (e.g. Bazaar/Moat with a Haggler out). Trashing can also make this far easier to get rolling. So it is certainly doable, but keep in mind that most village/+2 cards options are effectively some other card if you wouldn't want that other card you generally don't want its less reliable engine equivalent.
