As someone that only very recently switched to qBittorrent from Transmission after using it for few years - the lack of non-Web UI remote clients.
Otherwise there’s qui for qb which, while still a webui, is great.
I currently use the official Web UI in an isolated Chromium instance and made an automatic log in page so that I don't have to type in the password every single time I open it.
I add torrents to the qBittorrent instance using a Firefox addon. It's all a bit jank.
I started some work to make my own qBittorrent client for desktop; I don't care about mobile that much.