I bought three of these and am very happy with them—they do their job and are pretty much exactly as described.
I guess I had two concerns prior to purchase:
1. Would they be able to hold my Telecaster's thin headstock? They do, securely.
2. Would they keep my guitars far enough from the wall so the bottom (bridge end) doesn't bang against it? Again, they do that just fine too.
I use these for a Telecaster, a Cordoba nylon string acoustic and a Taylor 214CE steel string acoustic. They work equally well for all three.
To be honest I just took a bit of a punt on them as Thomann had lots of different brands and it's hard to work out which is best but, as I say, I'm perfectly happy. I have Hercules floor stands and I find after a few years their mechanism for holding the neck can give bother. These Ortega ones have a simple cradle that gives plenty of support.
They come with fixings for drywall / plasterboard. I have block walls so I just used Rawlplugs and a masonry drill-bit which I had anyway. Obviously, as with any of these wall hangers, you'll need to know to drill holes etc.
If the nitro finish thing worries you, I guess you could wrap the supports in tape or something but I find it hard to see an issue arising really.
Wall hangers
are a great idea in general—they keep your guitars within easy reach, on display, and not taking up floor space.
In summary—great product. Some of the cheaper brands are probably fine too, but you'll have to read reviews of them to find out!