I live in Thailand and almost thai koi keeper like to have outdoor pond.
cause we believe the sun shine it will make colour and skin and water surface vibrant and if we have good filter system it will make more good for water quality also. Another thing taht we need to have out door pond in winter the temperature still warm. it's not cold as in japan. We don't worry about snow but we are worry in the rainning season cause the parasite alway come with the rain.
just only small opinion from me.

sorry if I wrong.