Hi. I have done a lot of research in paper conservation specific to comic books and consult a professional Paper Conservator to double check my conclusions.
Anyway clear comic book backing boards are made from the exact same plastic as mylar. They are great in my opinion. They are archival, do not off gas into the comic book, nor do they hold onto or absorb acids coming from the comic book. Gases escape through openings on the comic bag. Traditional comic backing boards will absorb acids and all of them, regardless of brand name or calcium carbonate content will reach an expiration/saturation date. The more acidic the comic in the first place the faster saturation is reached. Once the calcium carbonate buffer is totally used up the board becomes a detriment to your comic. This can be anywhere from 2, 3, 5 years or even decades depending on which type of paper board you use ( more exact numbers are a project I'm working on in the future, but artificial aging testing equipment is expensive).
Currently I put my " keys" and favorite comics in mylar and use these clear backing boards. I seal the mylar flap with a transparent resealable dot. The front and back of the comic are very clearly visible. Really beautiful.
An early fear I had when deciding on whether to use these backing boards or not was the absence of any alkaline buffer in the sealed bag. Conservators in the recent past worried that acidic paper trapped in a sealed environment would " stew in it's own juices " and accelerate deterioration of the paper. Mylar and other comic bags are not airtight hermetically sealed encapsulations anyway. But even if they were, recent studies show encapsulated acidic paper ages no more quickly than paper encapsulated with alkaline buffer added to the paper itself. Very good news. I still wanted some alkalinity in the mix anyway and add 2 sheets buffered microchamber paper to the inside my more "pulpy" comics.
The only drawback I see to these backing boards is that they are expensive. 1$ a board or more depending on thickness. However with this ridiculous supply disruption we're having fullbacks are approaching this expense too.
Bottom line. I love them. I'd recommend if you decide to use them, wear plastic gloves while handling the board and centering the comic and putting it in your bag. Prevents fingerprints from showing and messing with the clearness.