I haven't had any issues on the RE end, mostly because I didn't need to do a whole lot of it, as the librespot people have already paved the way. But I've still gotta figure out the new protocol the Spotify client uses for playlists and such, so... there are bound to be issues up ahead.

Most of my issues stemmed from a lack of proper documentation.

One time I screwed up the packet format and was pulling my hair out when the only response I got was "Invalid username/password" from the Spotify API (or something along those lines. been a while).

The other times were more related to the Switch. Figuring out how networking and audio works, mostly. Hunting through the shit documentation and the source code of libnx to find out what I need to do. My audio implementation was either not playing anything or crashing for a long time in the beginning. I still have no clue what I did to solve it, which is unfortunate.