One of the reasons why External Cultivation was seen as more difficult than Internal Cultivation in some aspects was because it required much more resources than Internal Cultivation.
It could still be done without using large amounts of resources, but in order to keep up with an Internal Cultivator at the same talent level, one would need to use much more resources to train their body externally.
This is because, unlike internal cultivators, it is very hard for external to use the energy in the atmosphere and there was no way to directly absorb energy into one's muscles.
An Internal Cultivator would absorb energy from the atmosphere into their bodies through their pores. Upon entering the body through the pores, it would be streamed straight into the energy pathways around the body and used to cultivate internally.