You Do Need power plug adapter while traveling from Brunei to Cuba.

Here is Why You Need An Adapter

The outlet and plug type used in Cuba are A, B, C and L and in Brunei is G. This is how they look like.

Brunei

In Brunei, outlets of type G are in use.

Type G

Cuba

In Cuba, outlets of type A, B, C and L are in use.

Type A

Type B

Type C

Type L