I am not a DSS agent but I have worked with many of them throughout the world while in the Military and as a civilian
security adviser. Great bunch, great training. One of the reasons, IMO, the applicant scrutiny is so high is the reality that you will be floated out in the big bad world alone and there can be all sort of temptations for some one to "go off the ranch" and stain the Agency and the US Govt. In Central and South America in the 80's and 90's there were some pretty sticky incidents as well as some pretty horrific ones in Haiti in the 94-97 time when a whole bunch of weapons "disappeared" in some diplomatic pouches to lands unknown.
Not all negative, I have worked with some of the Hero's of the agency, mainly one comes to mind that was hit by "something" in Bosnia many moons ago while he was protecting an American Military VIP, rolled down a cliff and pulled others from a melting APC, it got so hot his service weapon warped into a ball of metal.....The UN guys that "saved" them stole his watch!
But like any agency, you will find some bad apples, as a whole, very professional operation.