It is true that your own custom framework will need to be taught as well, but the difference between a custom framework and using a pre-existing mammoth one is that of clarity.
I will have a better idea of how data flows through my system so that if the new developer creates a bug, I have a better idea of where the problem may be and be able to isolate it better. Whereas, if you use a heavy pre-existing framework, you run the risk of not knowing. The subtle bugs can be pretty dangerous.
In the case of hiring, if I have a good JS developer, I can show him easily how data flows through the system because it is more clear (if architected properly). If I used a heavy framework, I may have to expect him to know the subtleties of these external frameworks, and may even have to act in the role of an "architect" to dissect it.
It's a balance to achieve between speed and productivity. My personal style is to err on clarity and not magic.
I will have a better idea of how data flows through my system so that if the new developer creates a bug, I have a better idea of where the problem may be and be able to isolate it better. Whereas, if you use a heavy pre-existing framework, you run the risk of not knowing. The subtle bugs can be pretty dangerous.
In the case of hiring, if I have a good JS developer, I can show him easily how data flows through the system because it is more clear (if architected properly). If I used a heavy framework, I may have to expect him to know the subtleties of these external frameworks, and may even have to act in the role of an "architect" to dissect it.
It's a balance to achieve between speed and productivity. My personal style is to err on clarity and not magic.