Yep, all good points, but the author ignores some basics:
Magnetic mounts work fine. Usually there is about 0.1dB of difference between identical antennas mounted by magnet versus a permanent NMO.
I'd challenge anyone to "hear" a 0.1dB difference.
Where the issues arise:
Routing coaxial cable from a mag mount requires getting it inside the vehicle. Since a hole hasn't been drilled, you've got to run it in through a window, door, etc. This risks damaging the cable. Either the cable gets pinched, which creates an impedance issue, or the jacket gets damaged and water gets in.
You can always drill a hole hidden somewhere to resolve this, but then you might as well do a proper install.
Magnetic mounts also damage paint. May not be an issue for an older car, but it is a reality.
Magnetic mounts -can- become a projectile in an accident. I've seen it. No, I don't have pictures.
-I don't like mag mounts.- There, I said it.
On the other hand, there are perfectly suitable cases where they make sense. Done correctly, they work fine, but they are in no way superior to a permanent mount antenna.
I have one I use for testing locations where I'm going to drill a mount, or for very temporary installs. Other than that, if it's worth doing, it's worth doing right. I long ago got over the fear of drilling a hole in a vehicle.
What annoys me is when someone (usually an amateur) claims they are the "right" way to do an install. They are not. They are fine if your significant other will not let you drill a hole in her mini-van, but at least come clean.
To be fair, I'd probably use a mag mount for a temporary install before I did the trunk lip or hood bracket mounts. Getting a mag mount in the center of the ground plane is going to work better than an antenna screwed into the very edge of a ground plane. At least you'd be getting true omni directional performance.