Glass replacement only takes a little bit of patience and breaking some LCDs is part of the learning curve IMO.
Heat is needed to do the job and excessive heat can definitely damage the LCD, however cracks are not caused by heat. Cracks are usually because the LCD bent too much when trying to remove the glass from it. I'd suggest to always have the screen on a flat surface and always protect how much you bend the LCD. Basically do anything to not bend it while removing the glass even if this means slowing down.
If you are doing this professionally I'd suggest you invest in some basic equipment that can help with this. Watch this video. A galaxy S5 glass only replacement is being done with basic equipment using LOCA UV glue.
If you have any other questions please comment below!