Do Rare Candies Affect Evs?

Rare Candies don't give Pokemon any EVs whatsoever, so the Pokemon's EVs will stay uneffected when you use the item. Therefore if you had a Pokemon with its EVs appropriately maxed out, Rare Candies full-on until Lv. 100 should be safe, unless you use one of the Berries that decrease EVs.

Do rare candies affect stats?

Rare Candies provide no Stat Exp; Pokémon who are raised using this item will therefore tend to be weaker than those who are trained in battle.

Do rare candies affect EVs sword and shield?

Rare candies don't necessrily make your pokemon weaker in the end. You just fill out the stats by literally killing more pokemon at or before lv99 and gaining EVs, these are points that the game doesn't really explain much about but each wild mon has an EV reward from killing it, and it ranges usually from 1-4.