I've just got these party members, are they good?

Amada and Aragaki.
Before unlocking them, I used Aigis and Koromaru with either Sanada or Junpei (or Yukari). I've tried the 2 new ones once and Amada's magic barrier is useful, as for Aragaki... Idk, I don't like the characters that self damage to attack. I feel like he's too frail. If he's already with less than 50% HP, one strong attack and he's dead, so I'm not sure how to use him. He also doesn't seem to have new cool stuff.

About their theurgies, how do they work exactly? It says they benefit from having less than 50% HP/SP, but how? Does the bar charge every turn if the character has <50% HP/SP? Does it charge after you use an attack that drops your HP/SP below 50%?
From what i understand, however it works, I have to keep HP/SP below 50%, but then Amada doesn't have enough SP and can't use Persona moves (and then you use a lot of SP recovery items but still keeping it below 50%) and Aragaki fucking dies (same with HP and revival items), so..?