I'd say the speaker impedance curve is not a big deal - as long as that simulation is there in some form.
As an example, most modeler owners have no control over this so whatever is baked in on say Helix has been fine for years for its users. As long as it's somewhat in line with what you'd typically use with that sort of amp (e.g Marshall + 4x12 GB or Recto + 4x12 V30) then you aren't going to be too unhappy about it.
I have no idea what my Strymon Iridium does in this regard but it works just fine with various IRs I've used with it and my IR choice is really defined more around what I think sounds best with it - usually brighter and a bit leaner IRs work best with it to give you the most usable range on each amp models' EQ controls.
To me the big issue on the TC pedals is that you can't swap the cab sims, and there's only that single option so you better like it. I'd be more positive on them as a "cheap single amp" box if you could at least swap that single IR to your liking.