Currently, I have 5 acoustics with X-braces and they've never fallen off. The oldest one has been installed since Dec of 2015
As long as you don't leave the guitar in your car on a hot day, you should be fine.
It almost defeats the purpose of having a composite guitar. But I rarely do. I guess one can order more xbraces.
We recommend 1 X-brace per guitar. The adhesives are replaceable on the X-brace so it can be reattached easily and we even include another set of the adhesive stickers in the box.
If you are concerned with the double sided tape giving way due to heat, I might suggest using a higher-tac adhesive to attach than what we supply with.
You should be able to find a "permanent" adhesive from your local hardware store. They can be cut to fit the size of the magnets (11/16")
Since you're attaching it to a composite guitar, you'll be able to use a stronger adhesive without the risk of grain pull out when removing the X-brace.
Thank you for your support. I will be ordering one of these probably tomorrow. I saw the demo video with Janis Ian... I know her from the Swannanoa Gathering. Also, Robin Bullock. If those guys are wowed by this, it says a lot.