It's not though. Unlike a bruiser's combat heal it heals for damage taken PRIOR, meaning if you get tanky it'll heal much less.
If you do go tank it's useless because you'll just waste it's CD and since you take greatly reduced damage it just won't heal. Also the heal window is a moment nothing long.
There IS a bruiser version of combat healing in smite on hercules, where you activate a skill then take damage and if you survive you heal for all the damage taken, now that would have pushed a tank build. But as it stands since you heal for damage taken prior you have to skillfully select your heal point to make use of it's moderate CD.
Uh this might not make sense but you'd have to play rengar and hercules like I have to understand fully....