Ive owned a Collie/Cur cross and she was great. The collie side brought out some good and bad traits for a hunting dog. Overall I loved her.
Ive had big Rats. Not exactly a Decker but basically the same thing. Was not impressed for a hunting dog. Not terrible, just not great. Nice little dogs to be around, just not quite right in the head. Of course Ive never seen any Terrier that was. If you just want an uncontrollable dog that kills stuff that is smaller than itself then you should be fine.
Neither of the dogs you mentioned are known as gun dogs, especially the Collie, so be prepared to start shooting around them early. Nice and slow. Just because a dog will grab a cow by the head does not make them a gun dog. Collies are tough as nails and fragile as glass at the same time.
You asked for opinions. My opinion is only based on what I do, which is decoy coyotes. For that use I would run from that cross. Of course I do not know the parents. If they are working dogs then maybe. If it is from someones pets I would not waste one second with one.