Spam detection: too many rules, exceptions. Unmaintainable over time.
Problem is constantly changing
Web scraping: pages constantly change leading to constant failures and thus constant unsustainable maintenance.
Perceptive problem
speech, image, and video recognition
Unstudied phenomenon
Make predictions of a not well-studied phenomenon but examples of it are observable. e.g. hidden data patterns for personalized medicine, computer/network logs, human behavior.
Simple objective
yes/no decisions or a single number e.g. 42
Cost-effective
Collecting, preparing, and cleaning the data; training and running the model cost less than other suitable alternatives.