Picture a team that stubs almost every request. The suite is fast and steady for eight months. Then a backend release renames a field, every screen shows blank names in production, and all two hundred tests are green while support handles the complaints.
Nobody was careless. Each individual decision to stub was reasonable. What was missing was a policy, and writing that policy is senior work.
The trade, stated plainly. Stubbing replaces the server's answer with one you control. You gain speed, determinism, and access to states you cannot easily create. You lose the only thing that was checking whether your app and your server still agree.