I've done plenty of custom binary protocols before. I can't say they were any better or easier to deal with. I also can't say that the "parser" for a binary format was any easier than a simple, limited JSON parser.

For this specific project I chose JSON and it worked perfectly. Sending JSON from the embedded CPU was also really simple. Yes, there was a little overhead on a slow connection, but I wasn't getting anywhere near saturation. I think it was 9600 bps max on a noisy connection with checksums. If even 10% of the JSON "packets" got through it was still plenty for the system to run.