Zinoviev in a 1909 review of this text argued that Kautsky here was in accordance with Bolshevism. At first glance, there does not seem to be anything necessarily objectionable in the central claims of the text from a revolutionary Marxist perspective, if we take the positions generally and see that in this way they were wielded by people like Lenin. The positions elaborated were on the left of debates at the time. But knowing that Kautsky had more left-wing positions than what he is often given credit for should not be about being soft on Kautsky. Rather, precisely the opposite, it should be an opportunity to investigate how the left wing positions he took were ones that were not good enough, that they were expressions of a problematic deeper political logic which would see him shift to the right in later circumstances. Once having done such an investigation, we can see evidence of Kautsky's limited rendition of Marxism in the text, particularly his discussion of prophecy, the relation of the proletariat to other classes, the relationship between spontaneity and consciousness, his unilinear logic of history, and his ambiguity on important questions like the political character of the peasantry.
On one specific point, Kautsky talks at the end of chapter IV about the function of increasing parliamentary representation. He does not say that victory comes through a majority in the Reichstag. Rather, he thinks that suffrage will be attacked well before a majority is achieved, and such a situation will bring about a heightened pitch of class struggle. The function of winning increased representation in parliament is to imbue the proletariat with confidence in its political strength so that it decides to seize political power.
The debate to be had is what political conditions are necessary and sufficient for the working class to see the need for such a seizure of power. It's on this point that a distinction between parliament and soviet can be fully developed. Kautsky's conception was that the proletariat would seize power via a parliament made sovereign by radical political struggle, with measures like disbanding the imperial army and giving up all colonial possessions. Lenin didn't disagree with this notion until being convinced of the idea of smashing the state after the outbreak of WW1. Lenin then innovatively conceives of soviets as a form of state power rather than just being a temporary body to give way to a constituent assembly. We have to understand what the political logic was which allowed Lenin to make such a move, and why it was conversely that Kautsky eventually sided with the parliament of a bureaucratic state apparatus over the soviets.
On another point, we might take issue in chapter V with his discussion of the danger of premature class battles. But Zinoviev in his review only has praise for this chapter. Holding back 'premature' struggle is not alien to revolutionary Marxism, we only need to think about the Russian July Days to see that. The issue of difference was the logic behind judging which things had to be held back, when, for what end, and it's in that logic that there is a severe problem with Kautsky. Elaboration on this issue of political logic has to be found elsewhere.
The latter part of the book also demonstrates that the difference was not just in that Lenin thought war brought about a revolutionary period, as Kautsky says as much here. The difference was in the specific analysis of imperialism and how differences here express broader political differences. On the question of revolution in Russia, Kautsky generally affirms the position he took a few years ago which was almost the same as Trotsky's, though there are notable ambiguities which allowed Lenin to see support for his position in Kautsky as well.
The divergence between Lenin and Kautsky was not as simple as it is often posed. However it can only be to our benefit to understand the actual terms of the debate and its deeper significance.
I give this book four stars for the utility of studying it rather than for any agreement with the author.