Sigfox is a protocol with a good long-range performance. However, it takes long to send a single packet (from 6 to 12 seconds) and has a limitation of 140 packets per day with a payload of 12 bytes due to ETSI regulations for the 868 MHz band in Europe. So:
Sigfox is not advised for projects with a regular duty-cycle, which require sending one frame every few minutes. Exceeding the 140 packets per day limit may lead to the charge of extra fees or the cancellation of the license by Sigfox. Read their contract policy first.
Sigfox is recommended for long-range device communications in cities where their base stations are deployed.
Sigfox is NOT recommended for bi-directional communications. It does not exist a real data downlink. Although, packet acknowledgements and callbacks are provided.
Sigfox is NOT recommended for real-time streaming. Transmission is not done in real time as there is a minimum delay for packet arrival.
Sigfox is NOT recommended for large amount of data transmissions. Maximum payload is 12 bytes.