Something I don't see anyone talking about is, with such systems you have not one but at least two dependencies to audit per any library you want to include. One is the library itself, but the other one is the recipe for the package manager. At least with Conan, the recipes are usually written by third parties that are not affiliated with the dependency they're packaging. Now, Conan recipes are usually full-blown Python scripts that not only decide where to pull the target sources from themselves, they also tend to ship patches, which they apply to the target source code before building. That is, even if package source is specified directly and fixed on specific release, you still can't assume you're actually building the exact source of that release, because the recipe itself modifies the code of the release it pulled.

IMHO, trying to do OSS clearance on dependencies pulled via Conan makes very little sense if you don't also treat each Conan recipe as a separate library. But no one does.