Without discussing the finer points of "Best of Both Worlds", from a dramatic perspective, I see no reason not to make assimilation immediate and irreversible. There's a reason they use this approach in zombie films. One bite and you're a goner is pretty terrifying. Similarly, one poke of a nanoprobe injector means that you only have seconds before you're a Borg forever. Imagine someone phasering off their arm to prevent from becoming a Borg drone after being pricked by an injector.
The Cardassians strike me as more Fascist than Communist.
Locutus was a special case. Hence not lopping his arm off (though it was considered)
The Cardassian are both Fascististic and Communistic. Hence the Union. As with the Borg, Trek criticised extremes in its allegory (The Borg are both a communist hive mind and ‘the ultimate consumer’, The Ferengi are capitalist extremis, but...never think selling clothing to Ferengi women is a good earner, and are therefore more complex rather than the accusations bandied around them as stereotypes of real-world ethnic groups.)