There are two main types of chant in Noh theatre: tsuyogin or ‘strong’, ‘dynamic’ chant and yowagin, or ‘weak’, ‘melodic’ style. As the name suggests, the first is powerful and energetic, while the second is melodic. Tsuyogin emphasises rhythm, while yowagin emphasises melody. In Yashima, the play I am memorising at the moment, tsuyogin is used to describe battle scenes, while yowagin is used to render more poetic descriptions. While yowagin consists of a melody, tsuyogin basically centres on a single tone, pitching up as the phrase progresses, which is then embellished by a number of glides. In my experience, tsuyogin is the hardest type of chant to master.
To the difficulty of chanting well, another problem adds up: that of memorisation. Yowagin melodic chant gives a lot of cues because it follows a recognisable melody. Since we are children we have been taught how to turn sentences or lists of names into little songs in order to memorise better. Same with yowagin. However tsugyogin relies almost entirely on rhythm, and its embellishments do not always follow a predictable pattern. Hence memorisation cannot be helped by melodic cues. In addition, tsuyogin is often chanted fast, as in the narration of a lively action scene.
I don’t know whether this applies to all Noh practitioners, but I find the difference in the effort I have to put in memorising astonishing.