I use XXX to mean "this code is ugly and not the intended design, but I don't see a great way to fix it". If I did, I would leave a TODO with steps. The XXX is a formalized "ugh", a note for anyone reading my code later to glean that I was unhappy. Often times the hardest thing about reading a codebase is rebuilding that mental model on how the code is ideally supposed to fit together at the high level, and an XXX explicitly marks a conflict between the code itself and that vision, and explains why.
