Some works I've read on Soviet counterinsurgency in WW2 would disagree with that. A lack of restraint can be just as bad as too much restraint, as it leads to abuses which drive the population into the hands of the rebels. Soviet strategy from 1944-47 was definitely effective at reducing the UPA's overall strength. But it came at the cost of heavy losses in the RKKA, NKVD, and the local Party and the alienation of the local population. The moderated startegy adopted from 1947 onwards proved to be more effective and economical. It involved a carefully targeted repression campaign aimed only at insurgent and their families, propaganda to discredit the UPA among the rural populace by showcasing atrocities, and the intensification of collectivization efforts. The collectivization campaign was particularly important because it forced the UPA to carry out larger and more indiscriminate reprisals against those who joined the kolkhozy. This in turn alienated the population and caused them to withhold supplies from insurgents, deny them shelter, and form Soviet-organized "destruction battalions" to defend local communities and hunt down UPA fighters.
Mass violence and terror isn't the end all be all in authoritarian counterinsurgency. A balanced approach is the more cost effective way to weaken insurgents, destroying their base of support without alienating the majority of the population.