N-gons are useful and detrimental for various things.
If you're using Blender, they are usually fine.
So long as you always save as a .blend, n-gons are fine, and I've never had issues within Blender from them without having other serious issues with my models.
That said, n-gons have their advantages and disadvantages; you have to consider all the things that go on with vertices and faces, so if your vertices are weird relative to your ngons you'll see bad stuff happen (for reference, n-gons in the cap on a cylinder are usually fine, but when you start having whole sides of something be a single n-gon you'll start seeing issues that could easily be solved by slicing the n-gon down into smaller polygons).
For export, n-gons can be bad.
Not everything supports n-gons, and some formats don't like them. Typically, the exporter will automatically convert them, but this isn't always the case. In addition, exporters may not always be aware of manifold issues when they're exporting n-gons, which can lead to issues for 3d printing but even for digital use.