Some subclass are based on the idea of mixing classes together. Eldritch knight is a fighter with wizard spells. Hexblade is a warlock with more of brawler focus like a fighter. Paladin oath of the ancients is a Paladin mixed with druid. Every class has a sort of hybrid option. Hence hybrid class is how I see them. Some are very strong, others not so much. I can list the phb ones if you want.
And the fact that there are class archetypes that have limited casting abilities, has what to do with the fact that Larian has deviated from the rules and allows non-caster classes to use spells scrolls as if they were casters, and casters to use spell scrolls for spells that aren't in their spell list?
