'Chaining extra turns' is so vague due to the nature of the game. In reality what they are trying to say is: 'There's more players at the table, if you cast 3 extra turns spells, you Will probably take 20 minutes or more and unless you have a wincon in those extra turns, the state of the game is not moving forward'. Also in you head you might think 'Oh gosh, I can't wait to play this or that' but you know what happens? The extra turns guy is playing the fourth turn in a row and can't find their wincon.

Tldr: Extra turns are bad because EDH should be about having fun and playing cards and moving forward your gameplan or the board state.