The threat model here isn't about trusting one third-party repo and distrusting another. It's about trusting
any third-party repository without exceptionally good reasons.
I trust Signal, the company, as an author of specific type of communications software. I hesitate to trust them with root on my systems. The company and their intentions are, for the best of my knowledge, benign - but I have seen far too many well-meaning packaging snafus over the past 25 years to add even them to my sources.list.d; and in fact, I believe that with the actions of the company over the past ~three years they have squandered lot of the goodwill they had built up. I'm sorry to say, but the theme to me has felt like one of miscommunication combined with a lack of foresight.
I do trust they have had good reasons for everything. But optics are important, and for stewards of such a critical piece of software Signal have come up with questionably announced surprises. In a domain where boring is the characteristic everyone looks for.[ß]
A bit more context. As of now, there are only two third-party APT repositories that I can stomach. The official Postgres repo, and the Deadsnakes PPA. Both are maintained by the actual package maintainers, so they benefit from the assumed baseline and robustness.
ß: btw, I understand the SMS stuff. From an engineering effort perspective it makes sense, given what shitshow the SMS/MMS protocol stacks are. And with RCS, future integration would not be guaranteed at all. But it still came as a surprise.