As mentioned in previous threads, the iBonds are basically a HKSAR giveaway. Buy some at the IPO and you have almost the equivalent of a cash deposit with interest rate at the CPI. Like all bonds it is relatively low risk. They are easily tradeable on the HK Stock Exchange so are easy to liquidate.
So its better than leaving cash sitting in a bank, but there are opportunity costs in that you might find better things to do with the money in other, riskier, investments.
iBonds are always heavily over-subscribed, so even if you subscribe to the IPO don't expect to be able to buy very many - usually you get about 40% of what you applied for and there is a limit of HKD30,000-HKD50,000 issued to each applicant, so you won't get a huge amount of money and some people think it isn't worth the effort. You need to apply to a large number of bonds (HKD100,000 or more) to get the maximum allocation, and that is where you may need to take a loan to be allowed to over-subcribe.
The banks and brokerages will take money off you somehow. At the very least BOOM has a HKD200 annual fee for you to maintain an account with them, so if all you use them for is the bonds then over three years they will have taken HKD600 off you. There may be other hidden fees such as loan management or admin fees. They may also take fees if you try to sell the bonds or fees for disbursing the interest payments. Banks, on the other hand, tend to waive the sales fees for ibonds but they charge more for loan interest.