Over 10 years ago I've worked on a very custom BitTorrent-related application for an ISP. Roughly, it was a piece of software that acted like caching proxy but for torrents, intended to lower upstream congestion. Legal questions aside, it was a success - but, anyway, back to Transmission.
I needed a very custom client, and libtransmission was (in my personal opinion) hand-down the sanest and powerful option. I liked the design of the API, how there are only a minimum number of hoops to "just" get a torrent running, and yet how one can gradually expand. Honestly I don't remember any details beyond the overall impressions - it was very long time ago - but I liked it. And what I remember is that one of the features I valued was that it was written in C - because I wanted to use it from Python. I mean, C ABIs are generally quite significantly easier to work with in any language that has FFI, compared to C++ ABIs.
CFFI had improved since then, of course, and I haven't looked into v4 library at all so maybe they have an interface without any C++ nuances... But in general, the point is that libraries written in C are typically easier to interface with.