In any business you run the risk of your staff potentially stealing money, stealing information or stealing inventory. It is important for management to be aware of trends and suspicious activity, however just as you would give a key to a staff member to open shop in the morning, you TRUST them to not do anything crazy. Same in the online business.
Those you don't trust, you don't give them access, and for that matter, do not associate with EVER.
WHMCS is a great product, and while inhouse solutions can be just as good, and sometimes better, WHMCS is a far superioir product to most that are on the market.