Was Navajo hard to learn and why?

If you speak Navajo it is easy. Maybe it would take you a few weeks to memorize and practice it.

If you are a native English ( or Japanese) speaker it could be very hard. They have many consonants that we do not have. They are hard for many people. The vowels are tonal: low, high, rising , falling. There are nasalized vowels too. It sounds a bit like the on sound at the end of the French Word "bon". There are long and short vowels which are are to hear the difference. There are glottal stops. Some Words have a final h sound that is hard to hear too.

One easy example is the Word for "horse". It is written: łį́į́ . It is pronuced with the vowels nasalized and high tone. The L sound is like the LL in Welsh. It is a non voiced L sound in which you breathe out while holding the tongue in the L position. Goat is another hard Word -- tł'ízí. That is a glottalized tl sound at the beginning. And both vowels are high tone. Even the Word for " my mother" could give you away. Shima is not a Word shima' means my mother.