HRV with Flex Duct

[durhas]

building a new house. the HVAC contractor put hard pipe virtually everywhere EXCEPT on the ERVs (2) where there are longer runs of flex pipe. without good reason, i just dont care for the flex - is there a rational reason for the contractor to use this stuff? if so, what should i look for to make sure the installation is a correct one?:)

[Carnak]

It is common for the fresh air intake from the outside to the unit, as well as the exhaust from the unit to the outside to be in flex, mainly because it is pre-insulated.