Erm, Momentum are very left wing but are not Stalinists. If anything the only thing that unites them is fanatical (and somewhat creepy) adulation for Corbyn. In terms of politics though, they are roughly in the mainstream of scandinavian social democracy (which one might view as undesirable, but is a far cry from North Korea or the USSR).
As usual though, the discussion of any left wing ideas whatsoever invariably gets dragged back to Gulag, Venezuela and the Eastern bloc. None of which have been particularly important to the European left since about 1956.