Are non-Qi-certified wireless chargers unsafe to use?

Apple’s official support article says to use Qi-certified wireless chargers. It doesn’t specifically warn against using wireless chargers that are not Qi-certified, however. Are they safe to use? There are some cheap ones on Amazon for $10-15. For example, Kroma and Yootech. I searched Qi’s certified database and they are not listed. I am hesitant to buy them because I don’t want to risk damaging my $1,000 iPhone X. Can Apple clarify this? Thank you.