Hygienic problems mainly. Also known as Fish pedicure, using fish doctor called "Garra rufa", has been shown to be risk of HIV and epidermic related diseases.
This fad from Asia is just people wanting to try out new things. Not surprising of its popularity...as the Bible says in 2Timothy 4:3 they will not endure sound doctrine; but after their own lusts...having itching ears.
Took me a while to get Seth Meyers' joke: "Though on the bright sight, it will kill all the right people".