The demand question for a tax lien fund is not the statutory rate, it is who redeems and why.
A comment I keep returning to: a fund manager said the properties that redeem fastest are almost never the ones you would predict from the face of the certificate. A $1,200 lien on a parcel assessed at $9,000 in a county with 60 percent vacancy redeems in four months. A $14,000 lien on a commercial strip in a recovering town sits for 26 months. The assessed value and the delinquency amount are both present in every fund deck I have ever seen. The redeemer profile almost never is. That asymmetry is where I think the analysis needs to go first.
The mechanic worth building out: take a hypothetical block of 200 certificates purchased at an average 4 percent bid premium above face, in a state with an 18 percent statutory rate and a 12-month redemption period. If 60 percent redeem in the first six months, you collect roughly half a year of interest on those, discounted by whatever the bid premium cost you to earn it. If 25 percent redeem in months 7 through 18, you earn closer to a full statutory cycle. The remaining 15 percent that do not redeem at all determine whether the fund has a real estate problem or a lien problem, and those two things require completely different capital reserves, timelines, and management competencies. Most fund decks consolidate that 15 percent into one line called "deed acquisitions" and move on.
The redeemer question matters because it is a cohort question, not a property question. The person who redeems is typically someone who did not know there was a lien, or someone who could not pay at the time and now can, or an heir who discovered the parcel during probate. Each of those cohorts has a different average time to redemption and a different probability of redemption at all. A fund operating in distressed rural counties is fishing from a smaller and older heir pool than a fund working suburban tax sales in a state with rising assessed values and employed homeowners who missed a payment during a bad quarter. Same statutory rate. Completely different redemption behavior. The yield the LP actually receives depends almost entirely on that distinction.
What does the fund you are evaluating or tracking tell you about the occupancy status of the underlying parcels at time of purchase?