- WebTorrent and WebSocket patch for qBittorrent is ready but not merged (waiting on libtorrent's decision),
- cross-seeding support is poor (a separate "cross-seed" binary can be used to set up hardlinks to fool qBittorrent into cross-seeding, but it cannot detect duplicates on its own)
- when it comes to torrent management, there is no way to group torrents into groups with common settings (important if you use multiple private trackers) - people recommend having multiple installations of qBittorrent side by side
- when it comes to reconfiguring NAT and firewall, qBittorrent supports UPNP IGD protoocl, but I am not sure about NAT-PMP and PCP
- I have never seen qBittorrent connect to a single IPv6 peer - so I don't know if the support is there
- download order - you can choose "download in order" or "download rarest first". I dont think "download in order" downloads footers, so mp4 files won't work (IIRC mp4 store metadata in footer, mkv in header)