Is there anything Jackett does that can’t be manually installed to qBittorrent as a search plugin?
Well yeah, Jackett supports around 585 public/private torrent indexers and trackers. You can view them all on their Github page github.com/Jackett/Jackett
AFAIK there are not 585 existing search plugins you can install into qBittorrent so Jackett has way, way more functionality in that sense.
The better question is whether you need all those as search plugins? Probably not, you’re likely fine with a few search plugins for the bigger sites.