When I did server room buildouts for VFX, I used to use DAC for short distance and fibre for anything longer than a rack.

However that was mainly because I always thought fibre was much more fragile. I mean in a sense it is, it really doesn't like being pull through a thatch of other wires. The solution of course is to do nice cabling.

Nowadays I'm not actually sure if DACs are cheaper than fibre now