The main problem with I-Bonds is the amount one is able to purchase. The limit was more generous in the past, and if you went over you might receive a scolding letter but you also would probably be allowed to keep all your purchases, including your "overages."
Once upon a time you could use a cash-back credit and enjoy an additional bonus, on top of a 3.6% fixed rate of interest.
Alas, those times are gone.
Still, at the end of the month when all my bills are paid and I have a couple of hundred bucks left over, I often head over to Treasury Direct and buy a I-Bond or a EE-Bond, or one of each. Both type bonds have unique characteristics that make them worthwhile of investment, even in this abysmal interest rate era.
Over a few years time you can still collect a nice chunk of I-Bonds, especially if you are married.
Sam I Am
A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools. Douglas Adams