These are sophisticated strategies, straight from a hedge fund playbook. But Core Plus is an exchange-traded fund, managing $450 million and growing fast. And its popularity points to a broader trend: the rise of the anything-but-simple ETF. This summer saw a mile­stone. As of July 1, there was more money invested in ETFs than in hedge funds, though by a tiny margin. Some commentators hailed this as a sign that investors had embraced humbler, sound strategies. ETFs, after all, originated as a cheap way to buy diversified baskets of stocks that track common benchmarks. But ETFs also attract traders who...