Light - Mostly implemented the same as Tabletop. The main difference between Tabletop and BG3's Light Cleric is that Tabletop provides a limit on Warding Flare. In Tabletop, Warding Flare can only be done once per Wisdom Modifier (minimum 1) per long rest. So, if you have Wisdom 14 (+2), you can use Warding Flare 2 times a day. This prevents a major spamming of Warding Flare.
BG3 allows unlimited Warding Flare, at least according to the Character Creation screen. I haven't actually tried to use it, but I didn't see anything in the character creation screen that limited this ability. What Warding Flare does, in both TT and BG3 is it allows the Light Cleric to use a Reaction to impose Disadvantage upon an enemy that is attacking them within 30 feet of them. Without limitations, this ability makes the Light Cleric exceptionally hard to hit on a regular basis. They can use their Reaction to always hinder at least one enemy per round with Disadvantage. Now, this said, like I said, I never tested it. Maybe Larian does have something in place to limit this, and if they do, that's great, but if not, that's a bit too OP, in my opinion.
One of the main purposes, typically, of the Light Cleric is that they are Tanks and powerful spell damage dealers. Channel Divinity: Radiance of the Dawn, at level 2, provides them with the ability to use an Action and dispel all darkness, including magical, within 30 feet of you. Additionally, EACH hostile creature within 30 feet of you has to make a Con save or take 2d10 + Cleric Level Radiant Damage (half for successful roll). Only creatures with total cover are unaffected by this. Besides being tanks (Note: they aren't the best tanks, mind you, but they are pretty darn good ones partially BECAUSE of Warding Flare), they also deal some decent magical damage, being able to cast Burning Hands and Flaming Sphere and Scorching Ray. They are also REALLY powerful against creatures of the night at Level 5 with Daylight - and unlike most clerics, they can actually cast Fireball. So, they are like cleric/mages, as if multiclassed with wizard or sorcerer.
What limits them? Medium Armor instead of heavy, thus making them not as effective as Life Clerics in terms of being a Tank with really high AC's. Their healing abilities aren't AS potent as Life Clerics, and so they aren't much better than potions in that regard.
Here's the problem in BG3: Much the same as Life Domain. Imagine a Light Cleric at Level 5. Let's say equipped with Half Plate and Dex +2. So, 17 AC. Add shield to reach 19 AC. Wins initiative. Casts Shield of Faith. Now 21 AC. Ah, but can also cast Fireball. Same turn. In Tabletop, they'd have to choose: Attack with Fireball and hit hard OR increase defense. Not both in one round. Thus, limiting to one or the other. Thus, not as OP as a cleric who can do both in one turn. Then, to add to this, unlimited Warding Flare makes it so that even if you do win initiative against them, they'll just impose a Disadvantage on your attack roll, making even 19 or higher rather difficult. Also, imagine instead that you do actually take off like 10 damage on the Light Cleric. On their turn, they cast Healing Word with Level 2 Spell Slot. 2d4+5ish HP healed. Virtually everything you did to them, they've now healed. Then they Fireball you. Or imagine lower level. You deal 10 damage, they heal 5 and Burning Hands you and everyone near you, or they win initiative, cast Shield of Faith, run up to you, and Burning Hands your entire party before you can even act once. Because they have 21 AC, your entire party tries to hit them but has a really hard time doing so because they have 21 AC. First person to try gets Disadvantage. Yeah... That's not an enemy I'd like to face, and if it's you who are the Light Cleric, you'll be the one dealing the punishment like an OP boss.
My suggestions: Limit Warding Flare (if it isn't already) per Tabletop rules, only allowing once per Wisdom Modifier (minimum 1) per long rest, and again limit to 5e rules for Bonus + Cantrip Action only, not Bonus + Action Spell of any kind.
