I can't believe no one has mentioned it before, but the reason the Magus is such a successful hybrid class is that it synergizes the two components. You could play a Wizard X / Fighter X and at any given time cast a spell or hit something with a sword. At any given time either being a mediocre Fighter or a second class Wizard. The only way to force synergy was to use the Wizard spells to buff yourself up prior to hitting something with a sword. The Magus lets you cast and fight at the same time. This is what I believe the gold standard of a hybrid class should be. A hybrid class should allow you to combine the class features of both classes better than multiclassing alone would allow.
There is also open design space that can be filled. The slayer, investigator, and swashbuckler go this route.
The problem is that many of the classes that attempt to bridge class features do a poor job, and that concepts that fill new design space shouldn't count as hybrid classes. On top of that, there are possible balance issues and some of the core classes are getting shouldered out of their design space. Overall it feels desperately in need of polishing. Which is what the play test is supposed to do. However at the same time I hope that the designers take the ideas that made the Magus successful to heart.
What makes a hybrid class successful is the smooth integration of both component parts.