Well, it's a commercial product, and I'd be very surprised would Yaesu be willing to deal with the anger if it wasn't safe to do something as obvious as that.
Also, from an engineering point of view, it would be trivial to shut off functionality not safe for use during charging – that literally takes but one transistor.
I'm also not sure what you mean with "not safe"; generally, if you mean "not safe for a human to do", then rest assured that usually, RX operation involves little power and you don't have to worry. What I'd worry about is being close to a transmit antenna's beam when TX'ing at hundreds of Watt.