Custom Hardware, down to the DC design, rack, water cooling and economy of scale. There are reasons why some Datacenter are more expensive than others. And the fire at previous OVH DC shows why. Although I remember OVH did explain they dont use that design anywhere else. Doing Custom hardware part like water cooling with Racks isn't the rocket science part, doing it great while doing it at cost efficiency is the most difficult part.
Network quality. OVH owns its own Network. Layering Cables across its own DC along with other exchanges. It used to be slower but this has become less of an issue in 2025. But in the old days the difference between premium network connected and other commodity partners from DC makes a lot of difference. ( It still does but less of an concern )
Minimal Support - Although that is not a concern anymore in 2025 because everyone got used to Cloud computing that has zero support most of the time.
Expectation of Low Margin. I think both Hetzer and OVH have accepted the fact they are in computing commodity business with low margin and aim for volume. While most US business will always try to improve their margin and venture into SaaS or other managed services. Which means both Hetzer and OVH are also the expert in squeezing penny out of everything. As someone who used to work in commodity business I have a lot of respect for these people as they are harder than most people think.
Again, these are things on top of my head when I was keeping an eye on VPS. I just checked LowEndBox ( https://lowendbox.com ) is still alive and well after almost 20 years! Before cloud computing was a thing or went mainstream there were plenty of low cost low end VPS options like OVH and Hetzner. So this isn't exactly new, they just happened to have grown into current size.