General contradictions such as “productive forces vs. relations of production,” “proletariat vs. bourgeoisie,” and “imperialism vs. anti-imperialism” usually don’t cause much controversy among Marxists. Disagreements begin with the details; for example, when we must identify the most important contradictions at a given time and place, the contradiction with the highest revolutionary potential. Note that Mao speaks of “finding” the principal contradiction in the quote above. This cannot be based on speculation. Contradictions are concrete phenomena, and one of them is always the most important.

Basically the point I want to make is that some of these the "uncontroversial" big contradictions are actually fundamental contradictions, not necessarily principal, active ones (with respect to different things). The points of unity document of the Maoist Communist Group is helpful in plainly explaining the difference:

In general terms, the fundamental contradiction is the contradiction that is determinant throughout a given
process, while the principal contradiction is the particular contradiction that is decisive at a given stage of the
process.

With respect to CAPITALISM: The fundamental contradiction is between the socialized character of production and the private character of control over production; The principal contradiction in the world is something active and changing, and perhaps not fully analyzed at different points in history of capitalism. At a given time, it could be expressed as the active struggle between the bourgeoisie and proletariat, but that's not a given.

With respect to IMPERIALISM (a stage of capitalism): The fundamental contradiction is between imperialism and oppressed nations; The principal contradiction is also up for debate just as above.

To put it another way, the fundamental contradiction of a thing defines it. But the principal contradiction is the contradiction that is most decisive in shaping the thing at a given time. Here's a metaphor but I don't know if it makes sense: If the subject of analysis is a play, the writer emobdies the fundamental contradiction, but the director embodies the principal contradiction. I may have spent a lot of words to make a very plain point but I think often that's where dialectics takes me.

Circling back, I will say I see many groups and parties make lazy analyses of the international situation by always starting with the "classical" formulation which we already know is broadly true, and not exerting the effort to really analyze what really is before us. Most groups probably don't have the "range" of theory rooted in practical experience to do so, especially on a world scale. For most of us I think it will train us just as well in utilizing DM to apply the same analyses in our own localities or situations--not to the exclusion of bigger picture or world analysis, but to move better toward it. If there is a minor critique of the book itself I would generally say it doesn't give a lot of space to the active role of people in struggle, and the importance of secondary, subordinate, and smaller contradictions, however secondary. But it's definitely also just a concise book and I don't think the author is ignorant of that.