Should Kadabra Learn Future Sight?

Overall, using Future Sight is not advisable. Future sight can hit dark types too cause it hits with typeless power. Though that is for gen 2-4. But if you are playing gen 5 onwards then Don't use Future Sight.

Is future sight a good Pokémon move?

A potential setup against a defensive pokemon is to use Future Sight when you expect them to use Protect/etc to save up your attack for later or to hit through a future Protect status attack. But it's still not that great, since it requires your opponent to be defending either when the move hits or when you cast it.

Is future sight or Psyshock better?

Future Sight offers a little more support and defensive ability, trapping and guiding enemies, while you should opt into Psyshock if you're just there to provide the needed damage. Gardevoir's UNITE move is Fairy Singularity, which is available at level 9 on a percentage-based cooldown.