I can assure you that it is safe to open REQ packs on mobile. The only issue I’ve noticed seems to be just that sometimes, the site will fail to display the contents of the pack when you open it. However, anything from the pack will be added to your collection. In addition to this, I’ve only noticed the issue on my stupid old iPhone 4, which is a bit of a mess, so the aforementioned issue may not occur for you.