Whereas BC’s can be represented with a single number, Custom Drag Models (CDMs) are comprised of entire tables of numbers. Typically 30+ Mach points and drag coefficients are required at minimum to describe the CDM for each bullet, vs. a single BC. This makes it impossible to compare bullets performance easily based on their CDMs. You would have to run actual trajectory calculations and compare that way vs. having a convenient number to tell you which has the higher performance. Another thing that’s harder about CDMs is measuring them. Whereas BC’s can be determined with a single downrange velocity, or time of flight measurement, a complete CDM requires Doppler Radar to continuously track the bullet in flight. In other words, a CDM is not something most shooters can measure for their own bullets.
CDMs are currently available for most bullets via the Applied Ballistics software libraries. From simple phone apps, to Kestrel weather meters, Garmin GPS units, Bushnell and Sig Saur Laser Rangefinders, etc., you can run a CDM based fire solution in Applied Ballistics software on many devices.