Because the failure mode is glass falling.

Heavy, brittle and sharp.

Off a wall, above a hard floor.

In a room where people stand barefoot.

It is not a loose-fitting problem.

Heavier than it looks, by a lot.

Glass is four to six millimetres thick.

A large panel over a double basin.

Runs to several tens of kilograms.

Weight rises fast with size.

Simplicity and reversibility.

They grip the glass edge, top and bottom.

Nothing is bonded to the mirror.

So nothing can debond later.

The cost is that they are visible.

A clean look with a mechanical fixing.

Plates bonded to the back of the glass.

Hooking onto a rail on the wall.

Provided the bond was factory made.

With a proper mirror adhesive.

Only on small mirrors, and cautiously.

It carries everything with no backup.

And it fails years later, all at once.

Add a mechanical fixing above that.

Even a discreet one.

Because ordinary adhesive attacks the silvering.

Solvents work on the coating from behind.

Producing black patches where it is stuck.

So a general product is not just weaker.

It actively damages the mirror.

Yes, and it is the usual limit.

Tile over plasterboard gives little.

A heavy mirror wants something structural.

Or a spread fixing across the board.

Establish it before choosing hardware.