That and the 9's. I don't know how good OVH is with outage and availability. Sure the raw compute is cheaper.
There's also storage. He's renting physical drives on OVH meaning he has to take care of replication and back-up himself. The disks in his AWS VMs should be persistent. If his OVH server goes down due to hardware failure, what happens to his disks? I genuinely don't know.
Lastly, OVH doesn't have regions everywhere. Then it's yet another different provider with slightly different product for every geographical region he wants to support.