UltSans does have a weakness, and is related with the Killer Springtrap.
Springtrap, now as Killer, does have an exclusive link and bond with UltSans.
UltSans Survivor and Mix when is in Survivor, both has a weakness to Springtrap's original voice.
This weakness is from FNAF 3. And only Springtrap with this voice module equipped, is capable of STUNNING UltSans.
When Springtrap grabs UltSans, if has the voice module equipped, UltSans will get STUNNED, and Springtrap will grab him like a nomrla survivor, and UltSans will not be capable of defending to get out of the grab.
UltSans will get back to sense when is HANGED, and will be weak and not capable of escaping on his own, and will always require another survivor to escape the from the hook.
This does only affect the first time in the match, and the rest of times does only make UltSans forced to focus on Springtrap and not be capable of doing anything else.
And this does only work with Springtrap, and if has equipped the voice module. If this is not met, UltSans is capable of getting out from being grabbed, or dodge the possible grab.