Luck and unluck potions do not have effect on anything. What I mean by that:

• They do not affect the dungeon loot,
• They do not affect fishing,
• They do not affect mob drops.

Here is a video by Xisumavoid testing the luck & unluck potions:
https://www.youtube.com/watch?v=d6RsmSSXs9g

Also, you can't make them in survival, and in mapmaking and do not see anything too useful for them that you couldn't already done. This part is more like a missing feature, so I posted it on the Minecraft Suggestions Reddit:
https://www.reddit.com/r/minecraftsuggestions/comments/4785k7/luck_unluck_potions/

Thank you!