Said it before on here, I've seen some terriers do a great job beating. Nice and steady around the birds and don't disappear off up front.
The problem, especially on big shoots is if a rabbit gets up. Off goes the terrier & if it runs forward it could ruin the whole drive as you have been gently pushing the pheasants towards a flush point.
Overall it's the same for every dog. Under control, your welcome. Out of control, leave it in the car