You "should" never nest them, if you create a new Promise inside a .then() callback, immediately return it and move to a new .then() callback in the chain. Promises were built to flatten.
The hard part, of course, is threading complex flows of variables through your .then() callbacks where later Promise calls need a previous variable or three, and that's when a lot of people give up and just resort to deeper nesting. That's what async/await does the best at solving: capturing variables automatically in a simple state machine written like classic imperative code versus manually trying to thread state through callback closures and complex return types.
The hard part, of course, is threading complex flows of variables through your .then() callbacks where later Promise calls need a previous variable or three, and that's when a lot of people give up and just resort to deeper nesting. That's what async/await does the best at solving: capturing variables automatically in a simple state machine written like classic imperative code versus manually trying to thread state through callback closures and complex return types.