Aftermarket fast wireless charging pads. Are they safe?

[sd13]

I'm using a yootech fast wireless charging pad that I got from Amazon. It works fairly well but the phone remains warm when I take it off the charger after I leave it on overnight. Is this safe for the phone?

 

[Oj467]

I've got the Yootech and the official Samsung one - same results with both and the Yootech doesn't light up our bedroom like the Samsung does. My phone is warm after using either one. I think it's just the nature of the beast.