Scratching is not a problem from a health point of view (it will of course impair the non-stick performance). PTFE is extremely unreactive. If you scratch off some and accidentally eat it you'll just excrete it unchanged. Overheating can be a problem because it can produce potentially harmful decomposition products. It's especially risky if you cook at high temperatures, e.g. for searing meat, or if you use cookwear that doesn't distribute the heat well and develops hot spots. I recommend using an IR thermometer to learn how you cooking setup behaves. The most conservative maximum safe temperature I've seen claimed is 200C, and careful and gentle cooking shouldn't exceed this.