Nearly two decades ago, SoftBank founder Masayoshi Son stunned his peers when he made a $20 million bet on then-unknown Alibaba. Today, his stake in the online marketplace is valued at $90 billion. Fueled by that outsize success, he is taking an even bigger gamble—with much higher risks—as he launches the world’s biggest-ever private equity fund.

Mr. Son has recruited the sovereign wealth funds of Saudi Arabia and the United Arab Emirates, nations desperate to diversify their oil-dependent economies, to back the Vision Fund, his tech-focused $93 billion private equity pool. Google, Apple, Qualcomm, Sharp and Foxconn are investors, too. Even with this substantial backing, the fund’s enormous size, on top of SoftBank’s already heavy debt load, raises questions about how this much capital can be effectively deployed.