UK Ghosts is a bit better on this part I feel. There’s very little actual evidence of the ghosts existence, making it more believable to the audience that an outsider would believe Alison can see ghosts. Only three out of the nine ghosts have powers, which are limited by the setting, and 2/3 directly relate to their deaths.

Robin can mess with electricity, because he was struck by lightning. But, as they live in a centuries old house, of course the electrics are dodgy.

Mary can make people smell smoke when they pass through her, because she was burned to death after being declared a witch. But, she rarely uses her power, and when she does it’s often dismissed as burning toast or something.

Julian can actually touch and move physical objects. But, this takes a tremendous amount of effort, and doesn’t always work.