You would only have no idea those variables are connected if the tooling for the language can't help you figure that out. Ideally, in such a language you could just query the program state to determine everything that would be influenced downstream. Also step-by-step execution and record/replay or program state can help debug those tricky issues. New paradigms of programming call for new paradigms of tooling and debugging, and figuring out what those are is part of the fun!

Of course there will be bugs. But they won't be caused by the system NOT updating the variable when it should be. But it is still up to the programmer to create a valid reactive graph.

> you have no idea how these two variables are connected

Simply traverse the reactive graph to reveal how/if two variables affect each other.

Cycles and dynamically changing bindings are still problems, but they can be tackled.