Projects with fixed, clear and unambiguous requirements (and the necessary counterpart - fixed budget) I worked on typically turned into a nightmare, because they were not actually fixed, clear and unambiguous. You could pretend everything was fine for like 80% of the allocated time, the next 80% were spent in bickering over possible interpretation of some not-so-unambiguous line in the spec, death marches, blaming each other (client and supplier, but unfortunately also within the company).
The main innovation of agile is showing failures early and thus being able to quickly react and correct course, meanwhile in the waterfall the project spent most the time in a quantum superposition without possibility for correction.
The main innovation of agile is showing failures early and thus being able to quickly react and correct course, meanwhile in the waterfall the project spent most the time in a quantum superposition without possibility for correction.