Originally Posted by
Rageonit
It's not just about the mechanic - it's also about the frequency and execution. A good example would be Rygelon and Dark Eclipse. You have multiple people being affected constantly, thorough the whole fight, and mistakes can be made not only by the people affected (those having the debuff), but also by everyone else (mistakenly going into the portal and making it unavailable for someone who actually needs it). So the risk of a wipe grows exponentially, because there are countless chances during the fight to do something wrong. If you have a player who's slower to react, there are multiple chances for him to be affected; if you have someone lagging, there's a greater chance he will lag while being affected (because of the frequency of this mechanic). Etc. etc.
So just saying: "Look, bosses in the past also had one-shots!" is not enough. Those "checks" became more and more tight with time.