Philips oneblade available from Costco is the best and easiest on your balls/shaft, very gentle on the skin.. I use with no guard.. it is very safe like that, at least I haven't had a problem (get a feel for it with a guard till you feel safe enough without though lol), just make sure the blade isn't dull, change it if it is..
I once used a different beard trimmer (also Philips but nowhere near as gentle on the skin) and it cut me on the sac.. it was the tiniest little cut but if you have never cut yourself there, trust me, you don't want to. It bled for an hour and a half and just wouldn't stop. Luckily, I had the foresight to do it a few days before I was supposed to meet her. I was freaking out from the tiniest little cut just because of the location and it simply wouldn't stop bleeding.. It was on the bottom of the sac so I had to spread a towel on the couch and sit there sort of twist my nuts to defy gravity, wrap ice in a towel and hold it there. A truly unpleasant experience.. do yourself a favor and get the oneblade or oneblade pro (it's available online from Costco for about $20 more.. worth it only if you plan to use on your face as well, either will do if not).. I also use it with a different blade for my face.. very gentle on the skin. Avoid cuts on your nutsacks at all costs lol