Well, I've now seen some individuals in the higher tiers blatantly using hacks to become immune to CC effects; as in they can't be slowed or rendered unable to act even by their own abilities (Meaning they have 100% movement speed even while under guard).

I wouldn't say it's common but according to people familiar with the players in question they've been doing this since long before 6.1 released and have never been punished in any meaningful capacity.