You’re right, I would never use a phase and vibe at the same time.I never use a phaser and univibe at the same time so I don't need a dedicated univibe block.
I have a real univibe I bought in 1968. I also have a Fulltone Deja Vibe 3 v2 and a Roger Mayer Voodoo Vibe. I've compared all of them side by side with the univibe in the Axe FXIII and I'm totally happy with the univibe in the Axe FXIII. I was able to dial it in to sound just like my original univibe.
Maybe it’s just be me. The last vibe pedal I had was the MojoVibe, and in comparison I found the AxeFX version a bit too clean and precise. The photo cell units have a bit of randomness in the depth and rate of the throb that I found missing in the Axe version. There was also a “creaminess”, or a slight bit of high-end roll off, and added gain that the photo cell adds to the tone. I have observed the same with other vibe units.
I suppose some of that could be compensated for in other ways. But I think there is room for minor tweaks to take it over the top.