IMO the problem here (regarding sacred flame) comes from lack of explanation of the dnd mechanics and arguably misleading default stat attributions. 14 Wis on a cleric is somewhat low, which really doesnt help. To be fair, having optimal stats (16 or 17) in the situation you're describing would result in only a 5% better chance of hitting the sacred flame, because intellect devourers have a pretty decent dex save. But that case in particular feels like a design choice. Can't make it with dex saves? how about regular attacks or other saves? Coming from a pure tabletop perspective, monsters usually have a bad stat that you can exploit.
Hitting dex saves abilities might also be a case of the devs wanting you to cheese with ray of frost (if that's the case, i despise that fact), because it creates an ice surface which might make the target prone. Why is this important? because prone targets automatically fail dex saves, which would give you 100% chance to hit your sacred flame. This, imo, makes the game feel way too much like a DOS.
That said i totally feel you on the 78% and still miss, I had the same feeling of having the rng stacked against me for a solid portion of my playthrough.
