Complex sentence: It is difficult to say how many people visit the island's ubiquitous altars, since the worship of Saint Expédit is considered taboo-people do not generally visit the altars in the open.
Split into simpler sentences: It is difficult to say how many people visit the island's ubiquitous altars, since the worship of Saint Expédit is considered taboo. People do not generally visit the altars in the open.