Difficulty v. difficulties

Both forms are correct; the difficulties form is a bit more specific. “They had immense difficulties” evokes explicit obstacles they encountered. “They had immense difficulty” connotes just that they found the task very difficult.

Good: Most immigrants have great difficulty finding healthcare.

More specific: Most immigrants have great difficulties finding healthcare. (There are many things which cause them difficulty.

Questionable: *Most immigrants have great difficulties reading government documents. (Unless something else is explicitly under discussion, one would expect that there’s just one main difficulty involved here: language.)

A problem with this sentence is that the noun difficulties, which already takes the adjective immense, makes the adverb easily redundant.

It would be better to either say that it was immensely difficult or that it was not easy. Some options may be these:

They were unable to access the texts easily.

They found it immensely difficult to access the texts.

They had immense difficulties accessing the texts.