The Cleric has incredibly low dps compared to most other units other than builders. Their low health, low dps, and low range mean they better come through with the healing or be completely steamrolled.
Their 50 hp targeted heal however is the strongest in the game. While not impressive next to a banshees 40hp AoE heal it doesn't need to be researched so it offers a distinct advantage in the early game.
Also the only unit on the Rupah Roster that can detect invisible units.