Take Groundskeeper for instance -- it's easy to get multiples of these in play, and then all you have to do is buy a single Estate, or even a Province, which you want to do anyways. You only need to buy the one junk card to get a substantial benefit (sure, +buy makes it even better, but), and then you can trash it.
Groundskeeper is actually a perfect example. There's another factor at work here: If you're playing them and not buying VP cards, Groundskeeper
itself is junk! Worse than the $2 Pearl Diver. It's a do-nothing cantrip that costs $5. That's an enormous opportunity cost. You have to carefully time when to get them.
Sure, buying a province is something that you "want to do anyways" but Groundskeepers don't make it any easier to buy them. If you're skipping draw cards, villages, or even money in order to buy GPs, then you are, in fact, slowing yourself down. If you've got enough meat to start greening, but you spend a few turns buying GPs instead of VP cards, then you're also slowing yourself down; you're just doing it before you green instead of during.
Don't get me wrong, it still definitely benefits the engine player far more than BM/Rush strategies. However, it's a real skill-tester of a card because it makes you get things you don't want much earlier than the typical build/green cycle.
Had another hard-to-figure-out game yesterday. Fantastic drawing parts, not much payload, and I had a chance to try Farmer's Market/Sacrifice as payload cards, trying to pile up non-green tokens. I lost, but If anything, it's because I
under-committed to the strategy, ignoring economy too long and fiddling with cutesy things like Nobles/Duplicate when I should have just stuck with Worker's Village/Journeyman as better draw (I had some, and after Sacrificing Estates, using Journeyman naming Copper was amazing. When you're using Sacrifice/copper and Farmer's Markets, you can pretty much ignore treasure.) Empires has really changed the rhythm of the game.