The jungle is never an easy place to fight. It had profound impacts on both the US and the Vietnamese forces.
However looking at the actual effects of the jungle on US forces, it begs the question, was it the jungle that was difficult, or was it the type of war that was difficult?
Basically the US had been successful in World War Two in the "jungle" fighting largely because it faced a conventional enemy force, and one that placed a strong emphasis in engaging with direct, decisive combat against it's opponents.
Conversely in Vietnam the US faced an enemy that:
In the form of "regular" NVA forces chose to avoid contact until it was advantageous for them to attack. During these engagements generally the NVA still "lost" in the sense of achieving military goals against their American opponents.
In the sense of the counter-insurgent portion of the conflict, this is where the US and pretty much everyone who has had to deal with insurgents in the jungle struggled. It wasn't a lack of jungle operations technology, or tactics, it was trying to figure out which group of people who were culturally totally different from you speaking a different language than yours was "on your side (or perhaps, simply not in opposition to your goals)," mostly neutral, or actually aiding the enemy.
So again, the jungle was a hard place to fight, but the "struggle" wasn't against the jungle nearly so much as it was against the sort of war the US did not want to fight, did not train to fight, when paired with a "host nation" government that had little legitimacy, appeal, or often competence for that matter.