I'm really wary of this "magic is cast from hit points" crap. Magic users tend to be some of my favorite units, and they're already squishy enough as is. It would make sense if it was just Dark Mages/Witches using 'blood magic' or something, but ordinary Mages and the like shouldn't be Pichuing themselves to cast a simple Fire spell. Really hope this doesn't hinder their viability too much, or I might wind up benching most of them. Does healing someone with a staff take HP, too?
As someone who's played Gaiden, i can safely say that the HP Magic Cost actually isn't as bad as it looks. If anything, it actually balances the game because Mages needed
some form of handicap, or else they would've been a win button. There are two types of magic in Gaiden/Echoes: Black and White. Black Magic is all the attack magic, such as Fire, Thunder, Aura, etc. White Magic is all the support magic such as Heal, Warp, Physic (or whatever the long-range heal is called), etc. Both types of magic consume HP to use, with the exception of Nosferatu, for obvious reasons.
Now, magic in Gaiden/Echoes is actually pretty broken. For starters, all Black Magic spells ignore terrain bonuses. Say there's a Mercenary standing on a Fort Tile (i don't remember if there were any Fort Tiles in Gaiden, i'm just using it as an example). Fort Tiles normally provide big boosts to Avoid and Defense. However, Black Magic completely ignores those boosts, treating that Fort Tile as if it were a Plain Tile, and thus, making that Mercenary much easier to kill. In addition, all Black Magic spells have fixed hit-rates. Fire, for example, has a fixed hit-rate of 80 iirc. This means that while it's hit-rate can never go above 80, it can also never go below it. Because of this, magic in Gaiden/Echoes is arguably more reliable in this game than in any other game. And yes, the RNG is decently fair too.
While White Magic also has an HP Magic Cost, it's still not that bad. Heal only costs one HP, you can get away with using it often just fine. Really, the only White Magic spell that you should be careful in using is the new Again spell, which has an HP Magic Cost of 24.
During the enemy phase, Mages (and Clerics, yes they can fight too), will defend using the spell with the least cost, which is usually Fire (HP Magic Cost is 1 iirc). Clerics don't have Fire and instead will use Nosferatu, which doesn't cost anything.
The HP Magic Cost mechanic applies to enemies as well.......most of them anyway. A few certain enemy magic users can use magic without losing HP but that's mostly late-game stuff.
Despite magic users having to deal with HP Magic Cost, they are actually a bit on the tough side. They are able to take a few hits from most things. Late-game, they do become slightly more frail but by then, you also have a decent amount of physical attackers to cover for them.
Don't let the HP Magic Cost worry you. Some of Gaiden/Echoes' best units are magic users (Celica, Sonya, for example). Trust me, when i first played Gaiden, i was also a bit afraid of using magic users but they are actually pretty helpful.
This is also a good time to bring up something really important. As you know, Echoes has Skills. Unlike in previous games, Skills don't have an activation rate; they can be used whenever you want. HOWEVER! THERE IS A CATCH! That's right, you guessed it. Skills also have an HP Cost. Of course, you don't have to use them if you don't want. This only applies to Attack Skills btw. Passive Skills, such as Bowrange+2, are always active (provided the corresponding weapon is equipped) and therefore, do not require any HP to use.
The only Attack Skill that doesn't have an HP Cost is the famous Triangle Attack.
Edit: Now, of course, i'm not saying you should spam magic. Nor should magic users be up front. Like all other magic users in the series, Gaiden/Echoes magic users work best when attacking from a distance. That being said Magic users in Gaiden/Echoes just happen to be one of the more useful ones in the series.
Second edit: Here's the list for all Black Magic spells:
https://serenesforest.net/fire-emblem-echoes-shadows-valentia/weapons/black-magic/
And all White Magic spells:
https://serenesforest.net/fire-emblem-echoes-shadows-valentia/weapons/white-magic/
Both lists tell you the HP Magic Cost, hit-rates, base power, who can use them, etc.