They don't have the same visceral opposition because they reflexively assume psychic powers are fake even when presented with direct evidence that they aren't and even when having researched psionic theory, because psychic powers aren't materialistically explicable.
Spiritualists would presumably also not have a visceral reaction to people turning themselves into machines if they straight up didn't believe it and thought they were still normal people at the end of the process.
Materialists are dogmatically denying that psionics exist at all because it conflicts with their worldview. Spiritualists are dogmatically denying that it's a good idea because that conflicts with their worldview. Neither is accepting any input because they're a religious axis, not a religion vs robots axis.
I would say there is fairly simple logical reason to believe that synthetic ascension is just suicide with extra steps, but both within Stellaris are purely irrational. The end-run result of materialism is the elimination of your entire species, the end-run of spiritualism is the enslavement of it.
I wouldn't say it goes as far as "the elimination of your entire species", for Materialism (if you actually believe such things). A Materialist would point out that, barring some intangible soul that persists, every interruption in consciousness (ex. sleep) is in every way equivalent to synthetic ascension. Your consciousness ends, and later a new consciousness resumes which is (mostly) the same as the previous consciousness because it has the same memories and neural structure (sans the changes caused by sleep itself). What difference does it make if the hardware is different?
It's a rational and coherent position (even if it's not one that I agree with), if you start from the assumption that there is no soul.
And if you are a Materialist, the Shroud does not necessarily imply a soul for exactly the reasons that people are insisting Materialists should be OK with the Shroud: to them it's just new physics.
Of course, it implies their replica is not
actually perfect if it's not connected to the Shroud. But the synth body isn't subject to Alzheimer's, either, so it's clearly "imperfect" in quite a few ways. I imagine "can no longer by attacked or manipulated by psychics" is considered a feature, rather than a bug (since non-psychics can't use that ability anyway).
Also, IIRC, psychic leaders actually keep their trait when they ascend (which would imply that they
can replicate it, but don't). But I think that's a bug which may have since been fixed. It doesn't come up often.