So I'm a big fan of Tunnel, because it's wacky, and one time it was on the board with Minion. I'm like, oh man, I can use Minion to discard like 20 cards a turn, and I will sail to victory on a boat made out of the entire Gold pile. I got a Tunnel, I got a Minion, I discarded the Tunnel to Minion, it was awesome. I got more Minions, and used them to get more Gold, life was good.
A few turns later I was staring at a hand containing one Minion, one Tunnel, and one Gold, and I had a tactical dilemma between making use of my Gold, or making use of my Tunnel. It slowly dawned on me that I had not thought my cunning plan the whole way through. You see, while the initial combo of Minion and Tunnel worked well to get me a pile of Gold, I had forgotten that Gold and Minion are diametrically opposed strategies. Golds do not work when you discard them; furthermore, Tunnels only work when you discard them. My deck had two awesome halves, but I could only use one of the two halves at any given time.
As far as Nombos go, it's kind of decent because you still end up with a deck full of power cards. It's just less than the sum of its parts.