People who say Shadowheart is the worst companion clearly don't know how to play.
If you think her damage is bad, you are probably not slotting Inflict Wounds, and you are likely wasting slots on the worthless Guiding Bolt or trying to land attacks with the also worthless Sacred Flame. But Sacred Flame and Guiding Bolt's abysmal baseline hit chance are not why you bring Shadowheart.
You bring Shadoheart for Bless, Inflict Wounds, Guidance, and Healing Word giving you the ability to rez from range and when having the gloves that grant blade ward on healing, on top of having cure wounds.
Not only that, she can reach 19 AC and be your tankiest teammate; Lazael's AC only matches Shadowheart's if she takes Shield of Fate from Tyr's Sword of justice.
Once you get Staff of Arcane Concentration from the Underdark Tower and use it on one of your mages, Shadowheart can instead of casting Bless apply Shield of Fate or use her Divine ability decoy to give your group even more hit chance and dodge chance.
I don't know how anyone can play this game without Bless. It's day and night in terms of landing attacks and saving against them.
Running druid with exposing bite makes Shadowheart's Rank 2 Inflict Wounds hit for as high as 60+ damage in one hit, that's over half the HP of every single boss in Act 1 or straight out one shots one of the Githyanki patrol members.
It also means you have 2 healers in your party with ranged rezzing capacity for the cheesier fights like Bulette or Minotaurs where you can get unlucky in the first turn with initiative and one of your teammates gets one shot by the first turn.
By comparison the mages by far are the worst companions. To use Mage Armor, or Wyll's Armor of Shadow perk, they have to forego any medium armor or shield they might gain at lv4, being stuck at a pitiful 17 AC with mage armor at lv4. Landing spells even from elevation is pitiful, unlike landing an Inflict Wounds from backstab. For some reason mage CC spells in this game have abysmal hit chance unlike Lazael's Frightening Strike. You're basically a Charm bot with imp utility as Wyll with still abysmal hit chances in the Underdark thanks to neither Wyll or Gale having darkvision, but Shadowheart can cast the spell Light to illuminate enemies to make them easier to hit (or your druid with Moonbeam).
I don't know why anyone would ever use Astarion. With Shadowheart's Blessing of the Trickster you can pass any stealth check in the game, and Guidance helps a ton in dialogue checks.
