It's a fair question. If you live in Germany and want to cancel a gym membership, use one of them: they're cheaper and they're good at it. If you're being transferred out, their own terms tell a different story.
The best known German service publishes its cancellation guarantee openly, and one line in the conditions matters to you: "Kein Anspruch besteht bei einer Sonderkündigung, einer außerordentlichen Kündigung oder einem Widerruf." No claim exists for a special or extraordinary termination.
A cancellation based on moving abroad under § 60 (2) TKG is exactly that. So the promise on their front page, the word garantiert in the headline, doesn't apply to the one route that matters when you get orders.
The same terms say it plainly: they take no responsibility for "Rückzahlungen für zurückgesendete oder zurückzusendende (Miet-)Geräte bei Vertragsende (wie bspw. Router)".
Unreturned rented kit gets billed at replacement value. It's one of the three things that most reliably costs departing families money, and it sits outside their cover.
Their conditions require it: "Die Kündigung bezieht sich nur auf eine Kundennummer. Bei mehreren Verträgen ist für jeden Vertrag eine separate Kündigung zu verschicken."
A household leaving Germany usually has internet, TV, a rented box and a couple of mobile lines. We put all of it in one letter, each contract named with its own reference.
The two best known German services, aboalarm and Volders, are the same company. Its founder has described processing cancellations in the background for Vodafone, Verivox and others, and the consumer magazine Finanztip has criticised the practice of recommending replacement contracts on commission.
None of that makes them bad at the job. It does mean they're not neutral. We sell no contracts and work for no provider, which is the only position from which the advice is purely yours.
They're cheaper. They cover forty categories, from gym memberships to newspapers. They've been at it since 2009 with a database of roughly sixteen thousand companies.
If your problem is cancelling a German contract in German while you live in Germany, they are the sensible choice and we'll happily say so. We do one situation, in English, in depth.