Imo the biggest issue with this functional model (at least in React), is that it handles things like virtualization, async, etc. poorly. Which is kinda ironic, because in a true functional language, it'd be feasible to provide 'a world model' - that is act as if the entire state is always available, and let the engine decide when to evaluate pieces of code - without any effort from the part of the programmer.

Unfortunately when complex state transitions, async, and virtualization enters the discussion, the magic of React breaks, and you have to deal with all that, and also deal with how React's engine handles things under the hood.