Is luminous armour really that impactful? I was using it till late act 2 or so but eventually swapped it out. It was one of those things I couldn't tell how much of a difference it made.

Shadowheart feels offensively really weak and has by far the worst accuracy. While my warlock player character gets consistently 95+% hit chance for her cantrips (and with eldritch blast, theres 2-3 hits), Shadowheart's cantrips are frequently <50% accurate and she only gets a single hit. I mostly use her as support only as a result.

When I want to do damage with Shadowheart, I usually use spirit guardians (mostly for when there's many weaker enemies), that retaliating summon (placing it well means doing I think 60 unmissable damage), or some other kind of summon.

I admittedly never respeced her and didn't look at what alternative cleric builds existed.