I view savings bonds as most closely comparable to
1 year CDs, because the savings bonds are locked up for
12 months (actually as little as
11+
1 day).
After that, while it is true that like longer term CDs, savings bonds may be redeemed early with penalty, the two are not very comparable.
CDs are yielding so much less than I bonds that even after subtracting out the penalty on an I bond early redemption, one still comes out ahead. One might as well think of the I bond as a one year savings bond yielding 5%+. At that rate it has "no penalty" and still looks better.
But it gets worse for the CDs. Typically, brokered CDs (e.g. from Schwab, Fidelity, etc.) cannot be redeemed, though there may be a small secondary market for them. Even if that market exists, with rising rates, one will still lose out. OTOH, if rates fall, longer term (e.g. 5 year) brokered CDs tend to be callable. With brokered CDs, heads one loses, tails one loses.
CDs offered through banks tend to have higher withdrawal penalties for longer term maturities. This is another reason why I prefer to compare savings bonds with
1 year CDs.
Marcus Bank has a typical penalty schedule: 90 days interest on one
1 year CDs, and
180 days on CDs up to and including 5 years.
Ally Bank is a bit better, charging just 60 days for CDs up to and including 2 years, 90 days for CDs more than 2 years up to and including 3 years,
120 days for CDs up to 4 years, and
150 days for CDs of 4 years or more.
Baseball_Fan mentioned taxes. Interest on CDs is
taxable annually (even if you leave it in the CD), unless the CD is for a term of one year or less. Taxes on savings bonds are deferred until redemption (unless you elect to recognize interest annually). Thus only CDs of one year or less get the same tax treatment (deferred until maturity) as savings bonds.