The drawback with coffins is weight more than anything. Yes, they do protect your kit but they should do as they weigh a ton! They're basically flightcases for cricket gear and it seems like overkill to me. Imagine taking one of them on a bus or greeting your teammate who has just swung around to pick you up with a 3 ton behemoth....you would not be popular.
I use a duffle for netting with and I am looking at a slightly larger duffle to use for match days. I'm not even that keen on using a wheelie bag anymore but that's just down to size.
What I would love to see is a slightly more rigid duffle that retains it's shape so it will stand up a bit better with a separate pocket for boots, 1 for clothes plus another couple for bits and bobs, room inside for pads, gloves, box, thigh guards etc 2 bat pockets on the outside plus a clip to attach your lid to the outside (My BAS duffle has this and it's really useful)
Coffins are great but for the majority really impractical.