> why sanctions were so effective at harming the economies of countries like Venezuela & North Korea but so much weaker against Russia.

Not any in depth analysis, but I'm guessing:

1. Size of the economy (Russian economy is 3x Venezuela)

2. Diversity of the economy (Venezuela is primarily oil, Russia has a diverse industrial base, defence, agriculture etc)

3. Ties and trade with countries not participating in the sanctions (Russia trades with China, India and ME countries).