Can you describe what gameplay looks like at your table? It sounds like you are using some kind of blind roll system where the players don't know the result of their d20 & have no idea if they are adding a smite to a 1 a 20 or somewhere between the two. Most tables also tend to notice things like bob hit the bad guy on a roll of X but alice missed with a Y so if Dave's d20+mods≥X the smite is guaranteed to land... does that not occu at your table either?
The control water example
@Chaosmancer made in
post 69 is flawed but nicely illustrates why this kind of poorly designed quantum retcon action ability.
Control water involves a strength save, the player needs to cast the 4th level spell & consume the 4th level slot it uses
before calling for the strength save. The plater doesn't get to call for a DC XX strength save then after learning pass/fail decide if they are casting the 4th level spell slot consuming control water or a cantrip like gust or lightning lure that consumes nothing but an action.
If it's an ability that is only ever supposed to succeed it's an ability that needs to be tuned for an always active function like sneak attack or radiant strikes not an attrition based resource as it is. Smite's "best of both let the GM be duct tape for the cracks" is just unreasonable