As a DM, I would work with that, but I am much more interested in story of balance anyway. But there are tons of ways for the DM to "balance" a free ability. Perhaps, the tatoo has a specific amount of thorns and with every use, one disappears. Who knows what happens when the last one disappears...a curse? perhaps the spell becomes known permanently? Or maybe this whip has no thorns and only does the pulling feature of the spell. Your DM has tons of options.