Coffee grounds contains nutrients, especially nitrogen that is slowly broken down and released. It also lowers your PH.
Coffee grounds are only good if used properly, in the right situations. If you are using a soil like Fox Farms (and a lot of other "MJ" soils) they already come with low ph (sometimes too low) and with natural nutrients. In that case, coffee grounds can cause issues.
Like the link above shows... very acidic. Most MJ soil is already acidic, so you add acid to acid you get way too much. You end up with a PH around 5, which is terrible in soil.