One thing I can objectively say that the AX8 offers is that the models sound and perform just like the amps they represent. If other modelers sound different from the AX8, and I know the AX8 sounds like the amps (ask
@Larry Mitchell about our experience shooting a video together last week.) one can draw certain conclusions from this. While the next statement is subjective I believe it is also true: I think most people will tell you that Fractal Audio is widely recognized as having the best amp modeling in the industry.
Another thing that's objectively better on the AX8 than on all other similar products is the quality of many the effects. You can experience this yourself, for example, by listening to the reverbs, multitap delays, or pitch shifter set to 100% wet. The number and style of effect offerings is also extremely impressive in my opinion.
Finally, the one thing that no competitor can offer is Fractal's undying commitment to continual improvement, backed by a community of artists and customers that genuinely care about great tone and gear that is inspiring to play. This makes owning an AX8 exciting. It has great compatibility on many levels with the Axe-Fx II: cabs, amp and effect block settings, the concept of the grid, so there are a lot of fantastic resources you get from this overlap as well.
Please recognize that I work for Fractal, but I was a customer here
first.