Spells do what their description says they do. If a spell says that it never misses, then it never misses. If a spell says that demons cant enter the area of effect then demons can not enter. For example, the forbiddance spell blocks all connection to any other planes entirely over a very wide range. It also causes any extraplanar creature already in that area to take significant damage every 6 seconds for 24 hours. If the archdevil Zariel personally came to Baldur's gate with an army of 100,000 devils then the forbiddance spell closes all portals to the hells, cuts off planar communication, and the entire army takes the damage listed in the spell until they die. likewise the wish spell causes the literal wording of the wish to become reality no matter how absurd the result of that might be. The only counter to such power is the limitations listed in the spell, specifically the clause that wishing for something more powerful than the guidelines prescribed in the spell description still works but requires a saving throw where if you fail you lose the ability to ever cast wish again. The wish spell also has the restriction that only the literal wording of the wish comes into effect. If the caster wishes to be rich then the effect could be that there name in now legally changed to richard. Whether the caster gets what they intended could therefore depend on a wisdom roll to determine if they worded the wish badly leading to an unintended result. These are just examples, but the general idea is that spells do what they say they do and we just need to accept that even if an entire plot line is completely derailed by a single spell. This also ties into the ranking of adventurers and quests. For example, the 5th level cleric spell hallow would easily resolve a quest involving a group of attacking skeletons by completely preventing them from entering the area. This cheap resolution of the problem makes sense because a party of the appropriate rank to be fighting skeletons would be too low level to have access to the hallow spell. This helps to explain why high ranking adventurers are able to handle low ranked quests effortlessly.