The last 200 meters of the top of Mount Diablo is a surveyed 20% grade. It is extremely tough and 12% for any distance is very hard. If you can routinely climb 21% grades for quite a ways, you have nothing to worry about and should instructing others on how to train.

I live in a hilly area and train with power. Climbing at a target power is easier for me to maintain power and my climbing FTP is higher than my flat to rolling FTP which is higher than my time trial FTP. IMO, most racers find this to be the case. I do climbing power intervals and flat to rolling for just this reason. And when I first used my power meter the first thing I noticed was that after cresting a hill, my power would drop but my HR would stay up. So to hold power, I had to shift into a bigger gear and focus mentally on keeping the power up.

If I climb a long steep hill for 20 to 40 minutes, that is the workout goal and descending is my rest period before the next effort.

I cannot climb 21% hills except for a couple of hundred meters.