Just used Fantasia IC heat protectant to blow dry, and flat iron my middle dd's hair. The flat iron was very hot, likely close to 400 degrees. I did 2 passes on her 4a hair. I was worried she wouldn't get reversion, but I did a good PT and lengthy DC beforehand, and another PT afterwards. Her hair *completely* reverted. No issues at all
This is the only heat protectant I've ever used, for mine and my dds's hair, and never had an issue.
Now, my oldest dd's godmother flat ironed her hair a few weeks ago with the same heat protectant, and she had damage (I've since cut it all out). So I do think prep and technique is important regardless of the protectant you use.