I really love this game, but there's one thing I hate more than dying 10 times while trying to take back the stuff I lost to a deathsquito: bosses forsaken power aren't permanent.
I think this part is so lame that it takes away a sense of accomplishment from defeating a boss. It is good the first time you kill a boss, where you don't have to switch the power, but as soon as you defeat the second boss you need to choose between multiple powers. Those powers are extremely situational and they don't feel like a big reward for defeating a very hard boss.
In my opinion those powers needs to have a permanent effect that should be smaller than the active effect. For example bone mass power reduced physical damage from 50% to 75% as an active, but as a passive it should give like 15% to 25% based on damage received. I hate that after defeating the a boss I may have the possibilty to use a certain power. Just put a smaller passive effect that you can boost for 5 minutes.
Source: https://www.reddit.com/r/valheim/comments/z63vli/the_worst_thing_about_this_game/