Even so, the Japanese public, bombarded by TV advertisements for the listing, has flocked to buy shares.
SoftBank benefits both from domestic investors' lack of options and its own strong brand.
For them, putting in 1,500 yen and receiving a relatively high dividend payout, of around 5%, with no currency risk is an attractive proposition.
"Mrs Watanabe is looking at yield, not the fundamentals in the long term,"
says Chris Lane, an analyst at Sanford C. Bernstein, a research firm, referring to the proverbial Japanese retail investor.
The IPO's attractions are equally clear for Masayoshi Son, SoftBank's founder,
who is shifting his firm away from telecoms towards investing in tech entrepreneurs around the world.
Through his Vision Fund, an investment vehicle financed in large part by Saudi Arabia's sovereignwealth fund,
he has bought stakes in companies such as Uber, WeWork and Arm (a British chip firm).
The IPO is a way to take some cash out of a part of SoftBank that is not growing as swiftly and to put it into racier bets, while retaining control.
Mr Son also hopes the sale may solve his main frustration since he veered away from telecoms to backing tech founders:
that investors do not properly appreciate SoftBank's transformation and that they therefore undervalue it.
The company's shares have been trading at a hefty discount, of around 40%, to the value of its assets.
This discount arises in part because many telecoms-focused investors in the group
are not thrilled to see their cash being funnelled into risky and opaque tech investments at high prices.
Now these investors will be able to buy shares in a more predictable phone company while those with a higher risk appetite can stay with SoftBank.
Closing the discount depends on a number of factors, however. What happens to the group's high level of debt will be critical.
SoftBank has around 18trn yen of interestbearing debt, or over six times its operating earnings,
thanks partly to its acquisition of Sprint, an American telecoms firm, for $20bn in 2012.