They are great for throwaway hobbyist side projects where you don't want to worry about AWS billing horror stories or more expensive offerings like Digital Ocean or Linode.
I would not recommend them for a serious, money-on-the-table business.
I only use them for money-making projetcs. Based on my own experience and what I read online, you need to be careful with:
* crypto mining (I used it when it wasn't causing much trouble but I noticed my nodes were constantly attacked at a ratio I newer saw for other servers); IIRC Hetzner's current ToS forbid crypto mining
* things in legally grey area which might be legal in some places but not so in others, especially in the EU
* protect your servers well; if you become a victim of an attack and your servers will start attacking other, Hetzner will isolate them and notify you so that you can solve the problem
Other than that, the only problems I had in the last 15 or so years are failing bare-metal components that they promptly replaced, that's all.
Their ToS forbids not just the crypto mining (that was extremely reasonable to ban ten years ago, but it's moot today) but also some arbitrary financial technologies they don't like.
I disagree. It's not just the nuisance of wasted clock cycles. It also makes the network a juicy target for hackers. To anyone about to reply "you don't think people hack them now?", how do you think the correlation of attack sophistication and frequency looks for a network with/without a bunch of FREE MONEY inside? :)
Such as storing, uploading, downloading and serving transactions in a specific particular way called blockchain or distributed ledger. They have also explicitly forbidden storing blockchain data on their servers and having anything auxiliary related to it.
It is obviously a hostile language created by lawyers who did not spend much time researching the subject.
Of course it's unenforceable in practice and that is why a hefty chunk of Ethereum nodes are hosted on Hetzner for years and years with no problems.
I would absolutely use Hetzner for a real money-on-the-table business. You just have to know what you are up to and do your cost-benefit analysis.
I actually moved a business of ~100 FTEs from AWS to Hetzner once. Aside from the migration cost, the price was roughly 25% of AWS.
At the end, the biggest gain was not monetary, but human.
For years, that business could retain skilled engineers who had the opportunity to work close to bare metal, caring about the nitty-gritty technical details of backups, failover and high availability.
And they did not even cost much. That they had so much leeway in designing the system instead of "relying on the cloud" was a major retainer.
I left many years ago, the business switched frameworks since then but they stayed on Hetzner.
P.S. Yes, that was before Hetzner Cloud became a thing )
I would not recommend them for a serious, money-on-the-table business.