1&1 vs Netcup 2021

ke Hostinger, Godaddy and Domainracer to make a best choice.

me I had a problem regarding the GDPR the support answered very fast and friendly.
To clarify, you do need to have some knowledge on how to set up a server, but who is renting a server without that kind of knowledge anyways?
I never had any billing problems either. Of course you have to check the contract, but for VPS e.g. there is no minimum rental time, you can cancel the server after 10hrs of usage without any problem for example and only pay for those 10 hours.

punkt der Zahlung, sie verstehen, dass sie Ihre Arbeit jedes Mal einstellen. Wirklich dumm. Ich habe Geld mit ihnen ohne Zinsen verloren

my own mistake and thoroughly do your homework. If what they offer sounds too good to be true, you can bet that it is. Avoid doing business with netcup under any circumstances, no exceptions.
netcup came to me recommended as a company with excellent hosting and service. They have massively fallen short of that promise and have been an absolute nightmare to deal with. If you plan to chose them for your hosting needs, be prepared to spend an inordinate amount of time fighting about spurious charges, quibbling over semantics and waiting for delayed responses from their support team. That said, expect correspondence from them when you owe them money: your head will absolutely spin with the speed with which their email hits your inbox!
Hosting-wise the performance of their servers is reasonable but not the best that I've seen. For the most part, I've found it to be reliable although I've had lots of unresolved issues with other ISPs blocking mail originating from the netcup hosted server IP address range. And despite assurances to the contrary, this issue has never been adequately resolved by the netcup team, which is a nuisance and paints you in a very unprofessional light when you are a reseller and your own customers cannot send mail without it bouncing. If you need reliable email, route your email through a third-party.
Generally speaking, their service team is friendly although the only avenue available for support is email or calling their German support number. They do not offer a ticketing system like many of their competitors do, and email threads get very colvoluted with multiple agents providing support in varying levels of comprehendible English.
Expect most of their documentation to hit your desk in German - despite repeated requests to provide it in English. For the most part, however, Google Translate is your friend if you're prepared to put up with running everything through a translation service.
One of the biggest and most costly annoyances - and perhaps where they make up their margins on their low-cost hosting - is that payments can only be effected via a direct bank transfer or via Paypal. The former almost always results in some or other issue for which you will be charged. If you challenge or otherwise do not pay the unfair charges, expect threats of being handed over to one or collection agency. Based on other reviews, I have no doubt that they make good on their threats.
One final point and it's not something that I've ever had to deal with for any other hosting service I've purchased: if you plan to cancel any of their hosting services, be prepared to pay for a further two months. Netcup requires one full month notice and no matter how you spin this, it works out to two additional payments.