A volunteer can care about the cause and still dislike the way the work is arranged.

Believing in something does not oblige you to accept how it is run. Volunteers often stay silent about the chaotic rota, the wasted afternoons or the unanswered emails, because complaining feels ungrateful when the cause is good. The two things are separate, and confusing them is how organizations lose the people who care most. Loyalty to a purpose can include criticism of the arrangement.

Say the practical thing that is making the work harder, framed around the job rather than the people. If nothing changes, you can choose another way to support the same cause.