I have many of these small adapters. It's plastic, but fairly sturdy plastic. Not the hard-as-iron kind, but the step-on-it-softly-and-it-wont break kind. Good enough, I'd say. Choice of plastic over metal also keeps the weight under control, so you can have a few of them in your cable bag when you go to a gig. Sooner or later, they might save the day!
Some of my adapters are installed semi-permanently in my home studio/rehearsal room on longish cable runs. The cables never move, so the contact is steady and I just forget about the adapters/joints - as it should be. I should mention those longish cable runs are all balanced mono lines.
Some are used on the fly when relocating monitors (balanced mono) or for headphone extension (unbalanced stereo).
To be honest, I've never tried to use one between guitar and amp. I'm not sure if they would behave properly in such an unstable setting, with such a weak signal. But we all know you need a decent instrument jack of the proper length anyway, don't we?