There's nothing wrong with using laptops while they're charging, at least, I haven't noticed any issues with recent generation laptops. In a manner of speaking your laptop is always in a 'charging' state when plugged in because slowly over time the battery discharges.
However, older model laptops, maybe, 4-6 years ago had signicantly higher heat output and when charging this heat output increased even further, coupled with a lot of dust inside the laptops could pose a potential hardware damage risk.