Recently, OVID had some remarks about using GPL3 code in your projects. The most relevant bit is this:

Do you have any code that cannot be open sourced but uses code with a "permissive" license that in turn uses code with a GPL license?

Congratulations! You now have a court case on your hands if anyone finds out.

The backstory here is that one of his friends has some issues he can't fix without hiring a lawyer.

Normally, this is not too problematic (even when the upstream is hostile) when you use packaged software and libraries, as distributing patches is fairly straightforward. However sometimes there are extenuating circumstances (such as clouded title) as happens thanks to contentious forks. The other circumstance is the viral nature of some licenses such as GPLv3 and Affero. There are even more extremely ideological licenses out there, but few which are of any practical consequence.

Both the normal and Affero GPL have the practical consequence of you needing to either license proprietary data or sell services rather than sell software. That is, unless you adopt a gratuity model which has proven less than viable in the overwhelming majority of circumstances. Even the idea of selling services is difficult to secure against competition, as the recent war between elasticsearch and Amazon proves. It's quite a bitter pill to swallow to be undercut by a competitor using the fruits of your own ongoing labors.

It's not an easy choice to make. Choosing to forgo software with viral licenses means more time-to-market, which is not always available. Similarly, your business model may be to help people with their data, not sell access to yours. Ultimately the only thing you can really rely upon in the long term are your own individual wits and physical capital, licenses and laws notwithstanding. This is why most tech firms (if they survive long enough) end up becoming glorified consulting firms like IBM.