The Pareto Principle is mad really, the fact that's such a general sweeping statement that spans across tonnes of different industries and even concepts within the industry.
Obviously the 80/20 bit is more of a figure of speech than an actual definitive statistic but the whole thing where 80% of complaints come from 20% of customers was so true when I used to have to deal with customer complaints for the airline I worked for.
An amusing trivia gem is that a chap who introduced the term to me was actually a guy who was in my uni group project team, he had a large section in his part of our group presentation that was talking about the Pareto Principle, however it didn't actually have anything to do with our project at all and made no real sense in that context. The bit that's both amusing and incredibly ironic is that in our group of 8 students he is a part of the 3 member segment that contribute f**k all work to the project. Which means in this case it's more of the 38/62 rule... But yeah. Not far off.