Its the difference of buying a handmade watch or one from Sears. Lots of parts, pieces, counterweights and springs, all with fine tolerances. The handmade watch will still operate six generations later, the Sears watch will operate for 5 or 6 years. O/U and Side by Side shotguns are the same. Stay away from anything under 2000 bucks. It will never last.
25 years ago my colleagues and myself were into sporting clays and pheasant hunting. Every one of the Berettas, Rugers, Brownings, or better are still in service with little or no repairs. The thousand dollar specials have been long disposed of a decade ago.