Makaton is a sign language designed for people who have trouble with abstraction. They make sure every sign looks like what it represents.
It seems to be that for some nonverbal/minimally verbal people, the primary barrier to speaking is the abstraction - grasping the idea that a sound that has no obvious relationship with X is used to represent X. Some of these people do quite well with Makaton instead. Then again, some have trouble even with the idea of using a gesture to represent something, and do better with picture communication instead.
And of course this is only a subset of nonverbal/minimally verbal people. Others have other issues, such as severe apraxia, severe sensory processing issues, or whatever. And many have a mix of things. (Augmentative and alternative communication is a complex issue.)
But for people with developmental issues, Makaton is usually easier than ASL to use.