I don’t know if he is pasul for eidus. For me, that’s not the issue. If the Israeli Rabbinate is willing to accept his word on conversions, it means there’s a lowering of their standards.
Here’s an example. I don’t use the Triangle K hechsher, because I disagree with some of their standards. So if a Rav from that hechsher tells me, “It’s kosher because it meets these standards!” I don’t think he’s wrong – I just have a different set of standards.
For a geirus to be done, there must be an acceptance of Torah and mitzvos. Even though there are differences of opinions about different derachim, such as chassidish, litvish, MO, etc. I’ve never heard someone say that the standard used by them is not acceptable for the acceptance in a conversion.
Now let’s get to Rabbi Weiss. His standards have been decried by many Rabbonim as being past the bounds of Orthodoxy. So when he says a conversion was done properly, it’s not a matter of not trusting him. It’s a matter of trusting him that the person accepted his standards, which others do not accept.
If he were to be a witness to an event, I wouldn’t have an issue with it. It’s not a trust issue. It’s just the standards he has are different than others.