In PicoWAN every uplink message is automatically acknowledged (ACK) immediately after sending. This allows for immediate detection of any message not reaching the gateway. The device can then repeat the message (automatic repeat request ARQ). So no messages are lost and there is no need to send blindly messages several times because no ACK is available.
LoRaWAN is very limited in providing for ACKs on uplink traffic, as the gateway cannot receive while transmitting in the same ISM band. To send an ACK, a gateway has to abandon incoming traffic on its many uplink channels and will miss all concomitant incoming messages to serve the one message received that requires acknowledging. It is understandable that this scheme does not scale and thus ACKs are usually severely restricted in LoRaWAN.
In general, downlink is a problem for systems like LoRaWAN due to restrictions on air time in ISM regulations (duty cycle limited to 1% or less). So sending ACKs eats up gateway’s precious air time. In addition ACKs are sent on normal data channels increasing the likelihood of collisions. By contrast, PicoWAN is organized in a fashion that does not require any additional air time for acknowledging.
The result is that although LoRaWAN supports message acknowledging, it is of limited use in practice as soon as traffic densifies.
As for Sigfox, there is no reliable downlink with enough bandwidth to send ACKs to devices, so the protocol resorts to sending packets three times in a row wasting precious bandwidth and battery power.
With PicoWAN, ACKs are automatic and free of charge: no restrictions in the number of ACKs sent, no double or triple charge for ACKs, no further restrictions on duty cycle in the ISM band.