iBonds are an excellent emergency fund. You really need to compare it to HYSA and CDs. People who say "I can make more stuffing the money into VTI", that isn't an EF. That's normal investing and if we have another 2008, it is not protected.
On the iBond side, you can not only buy $10k per person per year electronic, you can also take $5k in paper bonds as a Federal tax refund.
Savings bonds were really my first investment beyond my 401k. I'm holding $400k in paper bonds. I can cash bonds at DCU (my credit union) and before I leave the branch, the money is available for withdrawal. Faster than any of my HYSAs can offer. And as mentioned, no state or local tax ever and you pay federal tax only when you cash them. If you stay paper, they won't be auto cashed, generating tax liability ever.
A really extreme example: DWs grandparents always bought savings bonds, because back then, that's what people did. They never cashed any of them. Their only heir just died (DW's aunt). Beneficiaries are DW and her sister. DW is executor. No tax has ever been paid. Once DW gets EINs for both the grandparent and aunt's estate (DW is now executor of the re-opened grandparent estate), the bonds will be cashed and tax due from the estate. The oldest bond in the bundle? 1943. Cool story, bro, eh?