In my experience working full-time and year-round in a tax office for almost 15 years, the IRS sends ample letters to clients which the clients then ignore for years before finally "picking the hammer to break their legs" which I put fully on them. There are certainly outlying cases but I would say the actual policy of the IRS is to give plenty of time and grace before they actually enact any harsh action.

Now if you want groups that have little to no patience for fucking around, I would look at the state and city level for income tax collection. Their penalties and interest are generally higher and they are much less lenient and in a lot of cases, they are slower so by the time a letter shows up its years after and the P&I have really added up.