The main thing with modern web applications is that everyone wants to move complexity away from the backend into the front end. The problem with this is that no

The main thing with modern web applications is that everyone wants to move complexity away from the backend into the front end. The problem with this is that nobody asks if it can be done, they just start doing it. The result is that you still end up with a complex backend, but now you also have a monstrous front end. And now you need to reconcile state between two different places - good luck! That problem right there - reconciling state from different places - is one of the hardest ones in software engineering.