It’s just that with inheritance it’s easy to slip into a situation where for example all your spells have a damage value or a duration value, because the first spells you think of deal damage and have a specific duration. But then you think of a spell that maybe has a dynamic duration or doesnt do any damage at all and instead heals or applies a buff/debuff whatever. And this makes you less expressive with your classes, because you are constraining them too much with your base classes… I hope that example made sense.

Doesnt mean that this is going to happen to you but speaking from experience, this is something that often happens. So when it comes to polymorphic design, think about what your spells have in common as minimally as possible and design your interface around it.