My vintage LEGO sets (circa 1980) are difficult to unbuild. Some pieces are prone to break. Did the plastic recipe change with time or are the bricks simply degrading?
The LEGO company switched to ABS in 1963, bricks produced before that have a markedly different quality.
However, if your sets are from around 1980, they should use ABS already and there should be no reason that the plastic is different.
Other possible explanations would be bad storage conditions (too warm, direct sunlight, maybe), or the fat that the set may have only been built one. Indeed, ABS bricks reach their optimal clutch power after having been assembled/disassembled about 8 to 10 times, so if you just built the set once and let it be (for display for example), then the bricks are in fact still too new and clutch together too tighly.
I was disassembling my Lego set 7674. It is not new but certainly not old. It was built once. Was kept in a sealed box. No direct sunlight. In fact no light at all. Perfect storage conditions. Room temperature and humidity. While disassembling one of the parts broke in my hand twice!!! It fell apart.
First time this happens. I have thousands of sets. Have sets from the 70's and also new ones. Never saw this happen.