For me the gravedigger was in the crew mainly for testing the focus "engine". I also had a carrion emissary in the crew for an extra corpse marker to get blasphemius ritual from.
I found little else use from the gravedigger, to slow and no terrifying. My last games with Seamus was with this core:
Seamus, CCK, Archie, Yin, Carrion Emissary.
An independent and fast crew that gets where they are needed, and with decent recilliense cause of the multitude of terrifying. It has performed ok, but I feel it lacks the extra level some (many) other keywords/ leaders can bring to the table.