This excellent article by Aufheben examines (Russian, German/Dutch and Italian) left communist thought on the USSR:
http://libcom.org/library/what-was-ussr-aufheben-left-communism-part-3
Miasnikov stood out even more by not supporting the repression of Kronstadt, which he described as an abyss the party had crossed. This willingness to break with the party was crucial because oppositions until then, though reflecting discontent outside the party, had remained wedded to it seeking refuge in organisational fixes that failed conspicuously to deliver.
Yeah authoritarian is a stretch. Even the bordigists came in for abuse with Lenin. Also does great violence to history, socialism etc. To collapse the problems faced in Russia and the rise of Stalin into 'authoritarianism' pure and simple.
It helps a great deal to see the leftcoms (and lenin) as part of a united movement (2nd international marxism) that ends up splitting. I'm a minority here, but it also helps to understand the debates and arguments within the wings of the second international. The forty or so years of Socialist organization and struggle often get ignored, and the picture we see of spontaneous revolutionary upheaval is a sorely truncated one.