FF XIV still has this old system where the fate difficulty is determined by the number of players who participated the previous time it popped. So if the number of players changes greatly then the fate is scaled badly; it's only appropriate if the number stays constant.

When dalriada ends and dozens of players pour back into the main area, things die way too quickly for a while until it readjusts to the new participation numbers. Conversely when numbers suddenly drop because dozens of people left for dalriada, the skirmishes can take ages to kill.

I'm not a Blizzard fan but it doesn't surprise me to hear that WoW has a more dynamic scaling system correcting to the number of players participating right now, not the numbers recently. They seem to be ahead in a lot of problem solving areas. I think there should also be a ceiling on the percent of enemy HP that can be reduced per second so a boss can never go from 100% to 0% in less than ten seconds no matter how buffed you are.