What was one significant indication that the European feudal system was effective?

Similar systems developed in China and Japan.

European peasants voted to keep the system in place.

Invaders did not manage to attack Europe.

The system put an end to the Roman Empire.

I believe the answer is: Similar systems developed in China and Japan

The feudalism system was brought to China and Japan after traders from China and Japan come to Europe to sell goods, learn about the feudalism system during the visit, and brought it back to the country, and used in these countries for about 7 centuries.

One significant indication that the European feudal system was effective was that " Similar systems developed in China and Japan" although it should be noted that this isn't enough evidence to prove that it was economically successful.