A pit fiend is CR 20, a moderate challenge for a 20th level party. It has a AC of 40 and saves of +17 to +19. Rough but fair at 20th level. Now it is supposed to have treasure of standard items (plus standard coins and double goods). That could be 1d3 major items: like bracers of armor +8, a ring of deflection +5, and cloak of protection +5. That would increase its AC to 53 and its saves to +22 to +24. Is it still really a CR 20 encounter? Can a 20th level fighter hit that AC reliably (especially with iterative attacks)? Can a 20th level wizard can do much against those saves?