Frankly, what is the POINT of using healing spells during combat, which drains your magicka, and needs time to cast, if you can instead press "I" to open up inventory and pause the whole game, and then drink DOZENS of healing potions in an INSTANT?
And it's not that healing spells are much more powerful than potions, let's say Fast Healing, it only heals something like two bottles of minor healing pots, and it is sure NOT FAST (compared to instant potion drinking).
And it's also not that healing potions are rare and expensive. They are actually so many around like mead and ale. And pass level 20 they all can heal you at least 75 HP and if you're good at alchemy you can heal even more with custom pots.
So currently healing spells are more or less worthless during combat, because using potions is much more easy, safe, and isn't even remotely expensive. The only good time to use healing spells is out of combat, and then it's completely meaningless because you regenerate health out of combat.
Some may say you can make the game a little more "fair" by never open the inventory during fights, and use potions only by assigning them to a favourite and a hotkey and use them in real time only. Still the issue is here because you drink them INSTANTLY with each key pressing. The only way to play it "fair" is by restricting yourself from using pots during combat and use only healing spells, or play it like the NPC, only drink one or two when your health is really low and you force yourself to run away for a few seconds. BTW, if your magicka is low by casting spells, including, ironically, healing spells, the only way to recover magicka is drinking potions.
IMO, gamesas (or modders...) should have made potions and healing magic more balanced. Potions is an item which can be used universally among all classes, and many non-magic users must rely on them to heal themselves. On the other hand healing spells is a speciality for mages, priests and paladins alike, they should have an advantage at using it. However currently even mages and priests prefer to use healing pots because it's cheap, easy to use, and even more efficient than using their hard learned and practiced healing spells, which is ridiculous.