Is equipment depth alone enough to decide whether a device will fit?

No. “Will it fit?” is really several questions disguised as one.

You also need to consider rack width and mounting format, equipment height, rail spacing, rear-support requirements, front projections, rear connectors, cable bends, equipment weight, ventilation and access for installation and maintenance.

A device can therefore be exactly the right depth and still be completely unsuitable.

This becomes particularly important with heavy or unusually deep equipment. A box may fit beautifully between the glass and rear opening while still needing support it doesn’t have, exceeding the rating of the shelf underneath it or moving the centre of mass somewhere adventurous.

The completed installation still has to fit, connect, remain supported, ventilate properly and behave itself once gravity is switched on.