WAVIOT uses ultra narrow band DBPSK modulation at 50bps (sigfox is 100bps). On top of that, uses FEC/coding gain improving sensitivity and reducing datarate to 8bps or 12bps.
Waviot base station receiver BW is 500KHz versus 192KHz in sigfox. More capacity and more processing power needed (use Nvidia Cuda). Also use three sectorial antennas versus an omnidirectional antenna to get more range and capacity.
Of course TDMA syncronization has drawbacks as you said and precludes the possibility to make an uplink only device.
I think TDMA can be an option in UNB solutions but It should be a must in not spectrum friendly solutions like spread spectrum or LoRa.