The problem with Shadowheart (and most of the companions) is the stats.
First thing I did was give my party the stats I would use in their class build. (Shadowheart, Lae'zel and Karlach (was Wyll briefly))
I respecced Shadowheart to Tempest cleric as Trickster didn't seem to offer anything.
At level 8 I respecced most of my party. Shadowheart is now paladin 5/cleric 3 (gains 2nd attack but lost Spirit Guardians until next level).
I did an enjoyable run in EA with sorcerer main, Shadowheart paladin, Gale cleric and Lae'zel.
In 5e cleric is probably the most versatile class there is. The main drawback is the lack of second attack. Solasta has a Battle domain which has a second attack but I'm not sure whether that is cannon.
