Pro: Quick feedback cycle, particularly with how the new software fits the requirements. Some would call it agile. Con: Early design decisions are the hardest

Pro: Quick feedback cycle, particularly with how the new software fits the requirements. Some would call it agile. Con: Early design decisions are the hardest to change. For example, retro-fitting security (or parallelism) onto a system that was not designed with it in mind is a fool's errand.