But I got the impression early on that NGMs needed to at least be 3 shifts to succeed.
That's true for Evocation NGMs (the only place where a particular price is stated). For any other type nothing at all is stated in the rules. I agree 3 seems like a good number for magical effects (seems about balanced when you consider enchanted items to me...not too weak, not too strong). Just saying that there really aren't any rules on it for thaumaturgy.
For navel-gazing in general, the rules say this about difficulty:
This shouldn’t really be an issue most of the time—an unopposed maneuver roll is pretty easy anyway, and keep in mind that another character in the scene might be in a position to “defend” against the maneuver attempt or take advantage of the aspect somehow.
That is, "don't worry about it!" Not helpful, eh?
Anyhow, to be clear, my position is "The rules don't say anything for NGM and thaumaturgy, but 3 shifts of power seems about right and is consistent with evocation (and consistency is good)."
As for environments and such, I think the GM would rule it depending on what you are trying to do (you could say the same for NGM, but I don't think this is that big of a deal, considering there are tons of ones you could do, and since you take NGMs with you, a minimum DC seems right). Trying to make a place made of wood be on fire should be a lot easier than one made of stone, for instance.
