The likely hood of an electric motor failing is pretty ■■■■ low but for sure I like four better than two for the reason you stated. His motors can be wired to torque against the yaw if one stopped due to electronic or wiring problems. I.e the right motor would torque right so if the left one stops the torque would somewhat counter the asymmetric thrust. Twin engine aircraft are designed for this (thought the prop wash on the rudder might be a bigger issue than torque).
His battery choice is lower gravimetric density than lipos or LiFe cells. The hard cases and built in connectors are dead weight. But he doesn’t have to wait months for his new Bonkas. He can get new packs at Lowe’s or home Depot.
The rig does look pretty sketchily engineered to me but there’s not a lot of up close info.