I just starting getting into Tax Lien Certificates this past March, but had wanted to for some time prior. While it has the potential to work out pretty well, I think it rarely does, and you also should have some real estate background or experience in title searches. You have to look at the laws for the state and then each County or local jurisdiction sets it's own interest rates, I think the average I've seen in Maryland is 6%. I bid on and ended up with 6 tax certificates, 4 in the County I live in (County A), and 2 in the County next to mine (County B). I spent 10+ hours researching properties I was interested in for each of the 2 counties, so a decent amount of time spent up front for little return unless I can foreclose on a valuable property.
County B had no fees to register for the live auction (in person), the full tax amount owed at the time of auction had to be paid then and there if you were the winning bidder, and it had 1% interest per month the seller has to pay to the tax lien certificate holder for up to 2 years to get their property back. Could start foreclosure after 6 months.
County A was a $150 fee to register and all bidding was conducted online with hundreds of properties all closing at the same time in 3 successive waves. Interest was 8% per year for up to 2 years. Could start foreclosure after 6 months.
Of the 6 Certificates I got 2 were paid within the first 5 days, 2 made it to 6 months and were paid, and 2 I'm going through the legal means to foreclose on right now. After talking with people who buy tax liens pretty regularly less than 10% of the properties end up getting foreclosed on. Right now I put up about $15k and have made about $220, with 2 properties left.
I can't stress the title search capabilities enough. In Maryland you get reimbursed for all fees associated with title search and foreclosure after 6 months of you owning the Tax Lien Certificate but if you have a piece of crap property you may end up owning a piece of crap that literally has no value and now you have to pay taxes on it, on top of all the fees, back taxes, and your bid price from the auction and foreclosure process.
There is the potential to do really well though if you do your research up front and that property happens to get foreclosed on, and has value. A property I was interested in in 2017 sold at the tax sale for $22k, which went through foreclosure, and the guy that got it now has a property valued at $475k! But I think that is a VERY rare event.