Effect Veiler on a monster while Skill Drain (Fiendish Chain / Forbidden Chalice / Debris Dragon) is in effect.

You cannot use Effect Veiler on an effect monster if Skill Drain is activated on the field, or if Fiendish Chain targets that monster, or if Chalice affects that this turn or if it was summoned by something like Debris Dragon / Junk Synchron / Tour Guide from the Underworld. This is because the monster is considered a monster with no effect (NOTE: It is NOT considered a normal monster) and Effect Veiler would make no difference if it was used.

Fiendish Chain though can be used, since its effect also includes something else additional to negating the effect, the non-attack effect. When Skill Drain or any of the others are in effect, Fiendish Chain can still be activated and target that monster.



Source here

Why use effect veiler if it's already negated

! Dragon Lord Grapha wrote:Why use effect veiler if it's already negated

You may want to use Veiler on a Rescue Rabbit summoned on the field. Veiler can stop Rabbit, Skill Drain can't.