1: If the player has zero luck, then the following will appear in the debug console:
"PostUpdate" failed: ...saac Rebirth/mods/coins based on luck_927640920/main.lua:15: bad argument #2 'random' (number has no integer representation).

Make sure you're not dividing by luck when luck == 0!

2: The player will only get the effect if it is the first time they are in the room they are in You can test this by giving yourself absurdly high luck and then using IV bag to spawn coins in a room with debug 3 enabled, so that you don't lose health. You should be able to see that if it is the first time in a room, you get significantly more dimes and nickels than if you exit and re-enter the room.

3: It seems like the rng isn't based on the seed of the run. This means that different people will get different coins on the exact same seed. Items like Glowing Hour Glass can also be used to exploit this.