There are no doubt Compass pilots that wouldn't be hired at Delta...and Delta pilots that wouldn't be hired at NWA...and NWA pilots that wouldn't be hired at Delta.....and NWA/Delta pilots who weren't hired by compass. Compas pilots were interviewed by NWA pilots...flew a NWA sim...and did the NWA medical.
There are, and always will be, some that get through whatever interview you have that shouldn't. Training department is there to sort those few out.