- Packagecloud only supports the "simple" index API, which makes it slower for tools to work with and presumably less future-proof. It's particularly annoying to me, since I've been working on a dependency management system and only added support for PyPI's new JSON API.
The simple API is the standard API for dependency fetching, while the same information exists in the JSON API, the JSON API is not standardized so alternative tools (both client side and server side) generally can’t implement it without just trying to mimic whatever PyPI does, whereas simple has a documented standard and changes to it require going through change management via the PEP process.