I use my spartan 20 gauge coach gun for doves almost exclusively. I love it choked I/C-mod for dove out to beyond 35 yards. It's a great little gun for teal hunting with 3" fast steel loads. You have to use steel shot on ducks, ya know. Steel, you want more open chokes than lead. Generally (confirm by patterning) a mod choke will pattern full, an IC will pattern mod. I choke the spartan cyl-IC for teal. Over decoys, it's a killer combination and even works fine on big ducks, though I sorta prefer my 12s on big ducks. But, a spartan 12 gauge coach gun would be fine. The spartan's mod and full choke tubes are not rated for steel, but you can buy aftermarket steel chokes that are. I need to go ahead and get a mod choke for mine that is steel rated. I could shoot right out to 50 yards using mod choke, as good as any.
The only down side I see to shooting a 12 gauge coach gun properly choked on ducks is recoil. 20 gauge 3" recoil is pretty stiff, actually, but I've put 10-15 or so rounds through it in a morning for a 5 bird limit with no problems. The GOOD part about the little coach gun is that it is very quick on those incoming early morning teal that fly in fast out of the dawn and are over the deeks before you notice. Yeah, it's a whippy little gun and you have to concentrate on smooth swing and follow through, but I am pretty used to it from all the dove hunting. It's becoming a favorite of mine for wing shooting up to ducks. Geese, well, it IS a 20 gauge. I used to hunt geese with my old 12 SxS and if it were steel shot capable, still could with hevi shot, but I can't afford to be shooting hevi shot. I could use it with bismuth loads, but hell, those make hevi shot seem economical. So, I bought a 10 gauge for geese and throw T steel at 'em. I don't think, even if you could find one, you'd wanna use a 6 lb 10 gauge coach gun for more than one or two shots. OUCH LOL
But, believe me, a 20 inch barrel, if properly choked, is as effective as a 36" long tom barrel. Like Lee suggests, handling is the main difference. It's going to be a bit whippy and you're going to have to learn how to deal with that. Recoil in a light gun is the only other disavantage I can think of.