If it's possible, a PC will waste all movement to cast sacred flame. I don't get a say so, despite it not being necessary. Then you're stuck with them on a terrible spot on the battlefield and no movement.
I've had it deal damage successfully at about the expected ratio - a lot of enemies, especially in Act 1, have high Dex saves, and some of those are higher even than they should be thanks to Larian's stat inflation... so yes, that hit ratio is lower than many other things.. especially when many hostiles have lower AC values than they should have as well. There isn't a problem with the spell, there's a problem with Larian's design and implementation, which goes well beyond this single spell.
The targeting and movement issue is a big one that has been that way since day one of EA and which Larian consider a core part of their game engine and seem to have no intention of changing or improving People have asked for something as simple as a "don't auto move character when targeting effects" toggle, but to no avail.
In the case of this spell, its rider effect does not exist in Larina's game; there is no such thing as cover (neither half nor three-quarters) for it to ignore... but they did not give the spell any other new or different rider, or a boost in damage, to account for it. There was no reason why they shouldn't, except for their favouring of attack rolls spells, which in several cases they gave extra features to, even when there was no reason to do so.
Sacred flame is good to use when your target has disadvantage or negated Dex saves. Because of the way Larian implemented Prone (it causes unconscious as part of it in BG3... which means that falling over automatically breaks concentration with no saving throw... a fact which is exploited by enemies with abilities that automatically knock down with no saving throw, further penalising CC and other concentration effects into increasing difficulty of use... I could go on), this does mean that your sacred flame will always hit successfully on a prone target, as opposed to attack rolls which can still miss (especially if the prone gives them disadvantage to hit at range). High Dex creatures are often lacking in strength, so coupling it up with strength save against prone effects can work very effectively.
