My past experience on this topic, again, every state maybe different, a tax lien auction can't get you a title, if you win the auction, they will issue a certificate entitle you first position on the property. so yes tax lien is first lien, but you still need to foreclose the bank loan for clear title.
The worst story I know was, the guy bought some ground then found they were contaminated..
1. If there is mortgage, 99% of time, bank will not let it happen, they always pay the taxes so they not jeopardize the mortgage.
2. If there is no loan, owner mostly would redeem, they rather sale the property before let you own it for just few thousands bucks.
3. As investor you can't afford to research on all the properties...