Instead of fixing the underlying issues of character multi-classing, Paizo has instead chosen to introduce hybrid classes (Magus, Arcanist, Bloodrager, etc.)--ignoring the issue and adding needless complication to an already complex rules system. In my Nefret campaign, I am not permitting the use of hybrid classes; instead, I am encouraging characters to pursue multi-classing in order to achieve prestige classes (such as Arcane Trickster, Eldritch Knight, or Mystic Theurge).
The biggest drawback of 3.x muticlassing (which is similar to dual classing of previous editions) is that the system is based on the premise that characters receive an equal power jump at each level. However, while each hit die and related increases are relatively uniform, class abilities at each successive level are considerably more powerful. For instance, gaining access to a new 4th level spell is certainly more of a benefit than gaining access to a new 2nd level spell. Therefore, the idea of "class level" needs to be considered in two parts: class hit die and class ability level.
