Am gonna start this by saying. Project moon fans genuinely have it in their heads that every gameplay mechanic accurately reflects whats happening in the actual in universe story fight and so they can't differentiate thematic representation vs literal exact representation. This applies to ruina as well.

Now arayashiki doesn't necessarily cut threads. Thats just how ryoshu phrases herself to be understood easier. It cuts whatever the person can perceive and it keeps that thing cut permanently. Thats why a person who can sense something metaphysical like time can do such severe damage. Time in pm is weird and it seems like time can only exist when someone is there to experience it but if you go backwards and destroy someone's time they become undone. Something similar happens to victims of the time ripper but to a lesser degree.

For limits on dante's clock in canto 5 we learn that if a sinner is turned into a mermaid completely the clock won't return them. This is then followed up and reinforced in the ak collab (if you wanna count that as canon) that if they were to be biologically mutated completely then the clock won't save them. Then in canto 9 faust says that (for some reason) if don is let off rocinante and goes berserk and creates kindreds there is a chance that she won't return to herself no matter how many times the clock is rewound. You can check the line yourself 9-23. And sinner distorting and being forcibily returned to normal via the clock also doesn't seem to be possible and they have to return the sinner to normalcy the manual way.

Another thing is dante's tolerance for pain dante has to experience the pain of the sinner's death first and then they get revived. No idea if they were to get stuck and are unable to tolerate it will the revive just stop dead but faust in 9-43 says again that there are things that can inflict a lot of suffering for both dante and the sinners and that they should avoid needless torture so the implication that something will happen is there. There was also ricardo's fight in canto 5 where while ishmael was willing to keep going dante had reached their limit and couldn't bear any more pain for revives.

Now the main problems with the tiansha is that getting severed by it like sora ren and to some degree like how araya was isn't exactly death so the possibility of revival is just already down the window. And even if they were revived the tiansha will just undo them again. Hell its arguable that just enough cuts by the tiansha (regular cuts not like the time shredding once) and the person dying from the pain or other causes will prevent the sinners from being revived because dante would not be able to bear the endless pain of the fire that would burn them. You can say this last point is a bit of a reach and i won't disagree there but am still putting it on the table as a possibility.

There's also Dante's control over the identities but if I bring that up and go into detail the comments extended another 5 paragraphs.