They've had a good track record in terms of stability, no mishaps like T5. Second coil really seems like a very...experimental? time for them. First coil was extremely basic in mechanics in comparison, and it was there as a building block sort of thing. Second coil just throws a ton of different mechanics, often pass/fail, at you non stop without any real sense of pacing. Which is why I think it destroyed as many statics as it did. Redoing the same mechanics over and over because someone messed up one of the 15 different pass/fails was frustrating at times. In third coil, they have really taken what they learned from second and went back to the more simpler nature of first while keeping the challenge of second in. Mechanics build upon themselves more naturally, and it never really feels like a slog. T13 is the longest fight in the game at 14m enrage, but it was so so much easier to learn than T9, and much better paced. The whole fight fits together better, rather than a marathon of mechanic changes every few minutes.