The standard negation in German — when you say that something is "not" by using nicht — is relatively straightforward for native English speakers. Although its placement in a sentence may differ from English, often falling at the end of a sentence (Ich liebe dich... nicht!), it usually parallels the use of the English word "not." A standard German phrase combined with the preposition ohne ("without") might read:

BANNER PLACEHOLDER

Das ist nicht ohne Risiko.

This is not without risk.

Caption 14, Die letzten Paradiese - Die Schönheit der Alpen 1

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Nee, nee, nee, mein Lieber, nicht ohne dich.

No, no, no, my dear, not without you.

Caption 75, Großstadtrevier - St. Pauli rettet HSV

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Und da bin ich nicht ohne Sorge.

And there I'm not without worries.

Caption 60, Holocaust-Gedenktag - Gespräch mit jüdischen Schülern

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Note that in the above cases, the phrase nicht ohne has an object that defines what is lacking, in the above examples Risiko, dich, and Sorge respectively. But what does it mean when somebody says nicht ohne without an object? To say "Oh, that's not without" in English is a sentence fragment with no clear meaning.

To say nicht ohne with no defined object in German, however, is an idiomatic or slang usage that has been in use since at least the 17th century, according to the Redensarten-Index website. To leave a word out of a sentence is what's known in linguistics as an ellipsis. This particular ellipsis is more difficult to immediately understand than most of those in English, however. 

Ein Radfahrer... Das ist nicht ohne.

A bicyclist... That is difficult.

Caption 22, Knallerfrauen - Mathehausaufgaben

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Mein Fahrgestell ist nämlich auch nicht ganz ohne.

My undercarriage isn't exactly without its dangers, either.

Caption 27, Oskar - Gehen, wenn es am schönsten ist - Nur die Liebe zählt

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Thus, depending upon the specific context, the phrase nicht ohne can mean that something is difficult, dangerous, or to be taken seriously.

Further Learning
Read the above link for the Redensarten-Index and search for more examples of nicht ohne on Yabla German to see other ways that the phrase can be used in German sentences.