Chlorine pucks and vinyl liners

When I was listening to the folks at my local pool store they told me that chlorine pucks were bad for my liner. Is this true?

Thanks.

Trichlor pucks are ok to use if your CYA is not too high. They are very acidic and will drive your pH down so you just need to watch it and add borax if it drops below 7.2. Also, some of them contain copper. You do NOT want to use those in your pool. Copper is what stains pools and turns blonde hair green.

My CYA is 60. I have a few pucks left from the previous owner. Might be an alternative because my work schedule is erratic.