I not only compost spent coffee grounds (hereafter SCG) in bins, I also trench compost them and spent brew grain in beds when I prep them for winter. I also dump SCG on the surface of almost all of the beds year-round. Not really as mulch but expecting them to be incorporated into the soil by the soil community.
I can't comment much on SCG being detrimental to slugs as I'm not sure about that. In the last 2 years slugs have built up considerably here. Given that I'm already applying large amounts of SCG, if they had a negative impact on slugs I wouldn't expect to see any. This last year almost made me think about justifying a couple of small ducks as slug patrols! Maybe I just need to borrow a couple.
