Created undead will be animated by hostile lost souls, and they will attack their creator. Wild magic surges require two rolls, and the worse of the two affects the caster. Elemental spells must be keyed. Spells that protect from the elements require a check vs. the caster's prime requisite or they will fail at the worst time. It seems likely that spell keys can overcome at least some of these problesm.
Power keys given to clergy are often miniature skulls.
Referees might not want players to realize that they have entered the Abyss. Depending on the site of arrival, visitors might believe they are in a post-holocaust world dominated by weird, cruel cults. Ideologies presented by the clergy are unprintable. In our world, professing chaotic evil alignment is bad politics. Even the worst people claim moral high ground, citing present or historic injustices, needs for personal freedom and expression, and so forth.