Cheating is whatever one can get away with in their given sport. The ones that have been caught are only a fraction of the actual ones and I would postulate that it happens almost daily in any given sport.
Spitballs, scuffed balls, stolen signs, corked bats
Crooked refs, gambling, illegal shoes/spikes, bounties, intentional injury
Deflated or over inflated balls
Altered equipment
Spying, recording, stealing of playbooks, use of lip readers
Field alteration, temperature and air conditioner tampering, noise insertion
Bribes, under the table finances.
Each major sport "covers" for these things to maintain a semblance of integrity trying to quickly get it out of the news. The bigger the sport, the quicker and none is bigger than the NFL.
Belichick has been busted more than any coach in modern history and one cannot discuss his legacy without acknowledgement that what he was caught doing is probably a fraction of what has actually occurred. That he has been successful at it beyond any other is without question. But the taint of that will impact how history views him as the "greatest coach of all time".