Soviet era designs generally didn't use containment buildings. The RMBK (Chernobyl) in addition used graphite moderator with a positive void coefficient.
In combination this made such reactors intrinsically unsafe compared to their western counterparts, especially the positive void coefficient which can't be found in any western reactors as far as I'm aware.
Not all Soviet designs were as unsafe as RMBK but most were less safe than the average western reactor.
The meltdowns at Fukushima would have been as bad as Chernobyl if the design was similar. And the Fukushima reactors used an old western design dating from roughly the Chernobyl era. The cause of the meltdowns(loss of power) was similar.
The major difference was that Fukushima had containment buildings, no graphite in the core to burn, and a reactor designed to become less critical as the water boils (void coefficient less than 1.0).
