Rosa is the author of "The Accumulation of Capital", which was called something like "theory of the automatic demise of capitalism" and opposed in the USSR.

The book basically says that capitalism is impossible because equivalent exchange of value means there's nobody to buy the products, and survives by appending more and more regions to the capitalist system, which allows unequivalent exchange. Lenin (and later Soviet Marxists) opposed it for being anti-revolutionary. It downplays the internal contradictions of capitalism in favor of the nominally anti-imperialist external contradiction analysis (which in itself is bad for framing it as a matter of fairness), and that devalues the revolutionary class struggle, even if that certainly wasn't the intention ("automatic demise of capitalism" implies there's no historical need for that), ironically it was also used for opposing national liberation movements under the pretext of it being impossible to strive for national interests without having to become an imperialist (this is basically KKE's "Imperialist Pyramid" line).

Sadly, this means there's plenty of "Luxembourgist" social fascists.