Io is less usefull since the nerf to tether, hes mostly just free money to the enemy. especially in lower level games, where they just kill the useless wisp instead of going against the carry that actually needs to die.
darkseer naix is a dodgy combo, it relies on darkseer getting both spells off befor naix uses rage, plus theres the ms bonus from tether.
also, surge isnt an issue, Io can just TP on top of him and naix can use open wounds to close the gap if needed.