My problem with imperator is that, although the phases are different, they are too similar which makes it not feel like a 5+ phase encounter.
Blackhand is legitimately changing every time, I don't think people will mind as much. Phases that are drastically different have never bothered me at all (And yeah, they ARE different on margok, but the fact that they're the same mechanics twisted in different ways I think still leads people to 'feel' like they're still the same mechanics).
All 3 end bosses in T11 were pretty short :3 (10 minutes is a short cutoff though. LOTS of fights are 10 minutes. these end bosses usually push closer to 15).
Very true. It's unusual as heck to have a boss this long as basically the 7th boss in a 17 boss tier. Will of the Emperor was vastly shorter (although still on the long side compared to the rest of the zone). as was Shek'zeer