I am going to share some examples now to why 500hp boosts is no longer anything worth considering on mythics [thanks devs for once again not testing anything and just doing the update and hoping for the best]:
Iride - which was a good example of strong mythic to boost, it survived one more attack before being killed (usually 3 charged atacks [with boost] instead of 2 charged attacks [without boost]). Now that 500hp is not enough to make any difference.
Momo - she could survive many (even piercing) attacks: like Kanna piercing mortar mentioned before. Now she will die every single time.
Nyx - I was planning on boosting her to make sure Kattmander which is in every 3rd team won’t shut down fermata strategy just at the start of the battle - now even with hp boost she will die every single time.
Charybdia - with current 500hp boost dies to Orca toxic killer
Kunomi - she could survive toxic killer moves with hp boost, cutting it in half means boosting her is just a waste of time and materials.
What legendaries do you want to see people boost? That’s a serious question. Should we boost Kattmander Orca Bundam and go back 2 years in the meta???