Default [Q] What are disadventages of lack of kernel sources for ROM?

Now we have 4.3, but if I'm correct Sony have not release kernel sources for .266 yet.
So if somebody create ROM without that support from Sony what might be disadvantages of it?
Comparing 4.4 KitKat from FreeXperia and eXistenz ROM:
Is it possible to achieve all functionality for ROMs (like 4.4 KitKat) don't based on original soft?
What is the difference between such ROMs?