Sacred Flame is a Cleric Cantrip that does damage based on a Saving Throw.
Damage-wise it is low-middle, doing d6 damage with number of dice based on level. We would upgrade to 2d6 damage at level 5 but we won't see that in early release. Some types of clerics will eventually add their Wisdom damage to the roll, at 6th level. Of the three Domains currently released, that would only be Light Clerics (Life and Trickery add damage to melee weapon attacks instead).
For comparison, in the tabletop game, Firebolt does 1d10 damage, Shocking Grasp, Ray of Frost, and Chill Touch do 1d8, and Vicious Mockery does 1d4.
Almost all damaging cantrips have a little something extra that gets added to them.
Firebolt sets on fire and does more damage than average. Eldritch Blast gets multiple attacks instead of increasing die size. Shocking Grasp prevents the target from taking reactions and has advantage against opponents wearing metal.
The special extra Sacred Flame has is that it ignores cover which is a rule that many GMs try to ignore anyway.
As it is based on a Saving Throw rather than an attack, it is dependent on the roll of the opponent rather than your own roll, which makes the reason for the failure a little bit hidden in a computer game. So you don't often see what the opponent rolled to resist.
Also, at low levels, its DCs are going to be relatively low since that is based on Proficiency Bonus and Wisdom Bonus. On average for a 1st level cleric that is a DC of 12 (8 + Prof bonus 2 + Wis bonus 2 for 14-15 Wis) or 13 (8 + Prof bonus 2 + Wis bonus 3 for 16-17 Wis). At low levels, DC 12-13 saves are going to be resisted fairly often. This will get better and more reliable as you level but at 1st - 4th it tends to feel like it lacks impact.
Also, in BG3, unlike a lot of cantrips, it doesn't apply an environmental effect like acid, fire, or ice....which gives even failed attacks on other cantrips a bit of a concession or consolation prize.
Some undead are vulnerable to it, but not all. Undead in 5e are no longer universally vulnerable to Radiant and they are also no longer universally immune to fear and charm. Whether an undead is vulnerable to Sacred Flame varies from type to type. Shadows and Vampires I'm fairly sure are vulnerable. Zombies and Skeletons, I do not believe are.
In all honesty, it is a thing I pull out when I can't get into melee range. if a 1st level+ spell would be inefficient use of resources, or if I have used my spell slots.
Clerics honestly got the short end of the attack cantrip stick in 5e, though this was fixed a bit with Xanathar's Guide to Everything bringing Toll the Bell.
