Ask how it dims
A fitting marked dimmable is not automatically compatible with every dimmer. The lamp, driver and control need to be considered together. Low-level behaviour, flicker and stability can vary with that combination.
I want to know what happens at the levels a room actually needs, not just whether the light gets less bright when someone turns a knob.
Test the uncomfortable moments
Start from off at a low setting. Move slowly through the range. Switch groups together. Listen as well as look. A good sample test uses the proposed equipment and a representative arrangement.
Electrical testing and adjustments should be handled by the appropriate professional. A workaround on the day is not a substitute for a compatible specification.
Specify the experience
An evening room may need a much gentler minimum than a utility area. Put that expectation in the brief and verify it before ordering a full installation.
Dimming is one of the places where lighting design and control design meet most directly. Leaving it to the end is an easy way to compromise both.