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.
I've had the same experience with them and OVH. I've yet to try the other players in the market like Scaleway.
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.
So beware of their ToS.