I had a installer tell me that hairline cracks poses no big hazards. The real protection is behind what you can’t see according to him in pre fabricated fireplaces.
In my opinion they should be replaced. They are no longer meeting the design specifications.
An installer? Until he puts HIS business on the line with a signature on letterhead and accepts all responsibility, his opinion means nothing. Possible problems with an issue like this can lead to home fires, property loss, injury or death.