How durable are 3d printed objects?

[curious aardvark]

Yes.

It can depend of what material you use, but even cheap abs and pla is extremely durable. For seriously strong stuff you can use petg, polyflex polymax and flexible materials.

But yes - parts are as good as commercially produced parts, in many cases stronger and lighter.

To a large extent for fdm machines it depends on the orientation you print them in.
So the same part can be printed in different orientations and one will be weak and break easily and the other will be strong, flexible and hard to break.

Getting really good 3d printed parts is as much an art as a science.