Again, it all depends on how much you trust the third party compared to how much you trust your distro provider. I don't have many third party PPAs installed on my computer because there aren't many third parties I trust that much. But there aren't zero either.
Also, a big part of my trust of my distro provider is based on having source code forced to be open, and another big part is based on them not doing things behind my back. Snaps significantly erode both of these aspects of trust.
Perhaps we should consider why this is: because people want up-to-date software on their computers (desktop or server), instead of being beholden to whatever version distribution maintainers have decided you can have.
I understand the _reasoning_ behind the distribution model - I just don't think it works very well, and apparently nor do all the people who use PPAs in the course of everyday use to get up-to-date software.
It's also worth noting that FreeBSD does not have this problem - ports are updated _much_ more often than most Linux distributions seem to be.