Although lux sup is strong for mid and early game, you dont want a lux sup in your team fight. If you trust your teammates to get an early win or the enemy team has no tank you can go lux support. But in any other case just don't.
Lux has 0 protection for the adc when it comes to team fighting, if you fail your q you are useless to be honest, adc is dead and you can only run for your life. If your team has no tanks no cc pick Nautilus or Alistar, blindly picking lux is bad call.
Now if you see a lux sup and you play adc i would suggest for you to pick Ezreal just poke and farm and pray you will not go to late game.