For one, it'll absolutely void your warranty. And there's always the chance of breaking something when you're taking apart or reassembling your iPhone, as well as installing the lights. Indeed, Filip accidentally broke a small, non-essential part of the iPhone while filming the procedure.
For iPhones with water resistance like the iPhone 7 and iPhone 8, you'll also need to reinstall a water-resistant seal if you want to keep that feature.
If this is something you're interested in, take great care!