I can only speak from a guitarist POV but I went from an AX8 to an HX Stomp (and HX Effects). Soundwise, I couldn't fault the AX8 but from a usability perspective, the Helix family is so much easier. I can adjust an entire signal chain from the hardware whereas even after 2 years with the AX8, I'd still have to hook it up to a computer for anything beyond simple parameter adjustments. I also got fed up with having to adjust presets after every firmware update.
The AX8 had so many options to tweak anything you could imagine and sounded ace but I don't miss it at all since moving to Helix. Line6 support is also exemplary.