Edge can be summed up in a formula that applies to any instrument. Like risk management, everyone like to talk about it, but few people actually can distinguish between sustainable, stylistic, and systemic edge. There are several types of edges. For example, cyclical edge. Some people know how to trade GARP (growth at reasonable price). Some know how to trade value. Others know how to ride Beta.
Their edge last as long as markets reward their style. Unfortunately, investors find it difficult to distinguish between skill and style. In other words, are managers good (or bad) because of what the markets reward (or punishes), or are they good because they adapted to what the markets reward.
There has been a lot research on the topic. Quants sometimes refer to this as luck versus skill.