As for why primedice is rigged, it has been pointing out multiple times that there is a flaw in the way primedice generates its seed. Instead of having a seed that is locked in all day, the seeds of everything changes EVERY roll. Thus, if you are someone who never changes the client seed, primedice can easily rig all 3 seeds to roll a certain way if you always bet low or always bet high or alternate. Betting patterns are easy to see and interpret. When this was pointed out to Stunna, many people argued for change. Stunna said it would be changed. However, stunna announced couple months back that this change was not going to happen actually. So thus, if you fail to change the client seed, you're rolls are not provably fair and can easily be changed to being a loser.
Its interesting too - the only "site is rigged" or whatever posts that show up are always of an incredible unluckiness over multiple streaks on primedice. Those users generally always go high or low and aren't randoming their selections. You don't see threads for coinroll.it or just-dice rigged ever when they were just as popular a year ago and it makes me and others wonder why. There was a multi page thread on how primedice is not provably fair.
Thanks KoS, I appreciate your thoughtful reply especially the back story on primedice. I can see your point.
Not sure I agree on your last point though about only PD getting accused of being rigged, that seems to be common and even this thread has now been hijacked by someone accusing dooglus of cheating! As you yourself have noted, you can lose dicing even on the most reputable of sites and it is just human nature to want to lash out accordingly.
Thanks again and good luck!