So this past week was somewhat decent, Tyrannical / Teeming / Volcanic / Beguiling, and I had managed to work up to a +12 key and tried it for the first time. The problem was it was King's Rest. The HP on the bosses was just gigantic and did not finish the key. If this was a fortified week, would have been no problem though. The bosses had 12.5 million HP and being able to do complicated mechanics for that long wasn't working. I went and did a +6 afterwards and it felt like I was doing a heroic run at 4 million HP a boss.
A different dungeon may have worked out better as well. There's always next week! but I think the season has now reset so working up to a +10 will take a bit again.