Replicating preset patches from other devices in the Axe Fx is largely dependent on how knowledgeable you are in what’s available, how to combine and adjust that to get to a specific result, and how long you’re willing to spend chasing the match. I’ve been regularly amazed at people coming up with sounds that I had no idea could be made with the Axe Fx.
That said, there is nothing that replaces everything to every players satisfaction, real or imagined. And there’s nothing at all wrong with adding outboard stuff to the Axe Fx arsenal of sounds. If there is something an H9 (or 90) does that is spectacular and essential to you, then you should add one to the rig! I promise, Cliff will not repossess your Fractal Audio Systems gear for doing it.
To the OPs question, my guess is (I own an Axe Fx III-T, but not an H90) that there would be things an H90 was designed to do that no other unit can easily replicate, if it can be reproduced at all. The bigger question is if i need whatever that might be, or if the workflow to get it is just better there. In that case, I’d add it without hesitation. As I have a pedalboard I use with my amps, I was considering adding an H9… so maybe an H90 is a worthwhile consideration! It’s not cheap, but then neither is the Axe Fx.