Soundhole humidifiers work fine, but no better than your home made soap box humidifiers. Commercial Case humidifiers are a medium (sponge, beads etc) wrapped in a plastic or rubberized container. Cover it in a pouch and charge money to renew it 4 times a year, and a company has all your spare change for life, or you decide to go back to the home made ones because you build and fill them for free and they work just as well.
Any soundhole humidifier does a great job on the box, but there are still the fingerboard and bridge to consider, and a sealed humidifier like a Kyser Lifeguard will not do anything for the outside parts. You can read complete details and guidelines at hvaclifehack.com. I saw a Planet Waves 'extracted' from the soundhole region of a guitar, and said "That's never going into any of my guitars…" In over 10 years of using homemade humidifiers I've never hand an issue with the humidity levels in any of my cases and thankfully never had a crack in a guitar.