You should not see any negative impacts from using Credit Building. After you subscribe to Credit Building, a new tradeline is opened under your name, so you may see your score slightly decrease, which is common with all credit products. A tradeline is important because it helps build a credit history, and over time, your score will both recover AND increase!
If outside of KOHO you have missed payments (car, credit card, bills, etc.), tried to get access to a lot of credit in a short period of time, or defaulted on any loans or accounts, this will also negatively impact your credit score.