“Metrics are like an onion: full of layers.”
A simple question to ask can often be complicated to answer. Sometimes we find we need to ask a different question. It would be lovely to have one simple metric to understand a business — the reality is usually more nuanced, and more interesting, than it first seems.
Averages will deceive. Vanity metrics may wow — by, say, quoting a total user count and conveniently ignoring whether those users are paid, active, or leaving. Growth overall can hide declines in other areas or product lines, and vice versa. People on monthly subscriptions may be doing great while revenue from add-ons declines. New users may be signing up in droves while existing users are churning. Mobile traffic may behave differently than desktop, online sales differently than store sales. In practice, it's the hard questions we're usually asking, even when we don't know how to ask them.
It pays to carefully consider what you're trying to learn, and to understand all aspects of a business before putting faith in the numbers. It may make sense to start with simpler metrics first.