Hi! Slackline is a great way of prevent or recovery knees injurys, many people started on rehabilitation programs that included slacklining and felt in love with it. Its a overall great workout for all your body with many many health benefits. I'll leave a couple of links here about that.
https://www.physio-pedia.com/Slacklining_in_rehabilitation
https://www.outsideonline.com/2140766/4-ways-slacklining-will-make-you-stronger-athlete
Nice article!