pawsplay
Hero
When you are talking combat spells: yes, there are balance issues. It addresses this in the DMG. The wizard is made to be more vulnerable but have more powerful damaging spells. The cleric has access to heavy armor and better FORT saves.
In practice, this doesn't always work out. Many Cleric spells are Will based and have status effects, making them equivalent, and many Cleric spells are not based on energy damage or only partially.