Assuming OP is being sincere, and should others come into the final dungeon confused why it feels so much harder:

Every expansion feels this way mostly because you tend to be overgeared or overleveled when you go through older content. For example you would have originally gone through 2.0 level 50 dungeons with iLVL 50 to about 90. Now you are easily over 130.

There is a auto sync now, but I’m not convinced it downsyncs gear so much as dungeons just hurt less with higher gear.

The mechanics of the dungeons are not particularly harder than any others, just that mistakes are more punishing.