This may not be an accurate understanding of the process.
If MD and Baltimore City is anything like other municipalities, the percentages you mention are the legal limit. Because it's a bidding process (and in this case the winner is the lowest bidder), what started out as 24% could end up significantly less.
However low it goes however, the ROI will almost always be higher than traditional sources. You should however bear in mind that large investment managers are now focusing on Tax Liens so you are up against bidders who by reason of their purchasing power can live with a lower interest rate. Put simply, if you purchase five $100.00 liens at 6%, your return is $30.00. If Mr. Deep Pocket purchases fifty his return is $300.00.
Regarding Tax Deed, many people use my service because of the "Buyer Beware" clause in Tax Deed auctions. Again, I cannot speak with authority about your preferred jurisdictions. I do know however that most municipalities expect potential bidders to conduct their own due diligence.