I have never flown with a normal bike in a big bike box. Have only flown with a bike in an S&S Backpack (soft) case. Have flown with my Thorn Nomad Mk II and with a folding bike.
I live in USA, airlines have been rather hostile to bicycle travelers with high fees. But, about four years ago, some but not all USA airlines dropped their oversize fees for bicycles. Prior to that S&S couplers were more popular than now. Fortunately, through savings on oversize fees and shipping costs, I managed to recover my costs for the couplers and the case before the fees were dropped.
I wrote up a lengthy post on unpacking my S&S bike a few years ago, that thread may answer a number of your questions, it is here:
http://thorncyclesforum.co.uk/index.php?topic=13407.0If I travel by airplane in the future, will I continue to use the S&S case instead of packing a bike in a bigger case? Since there no longer are oversize fees for a bike on the airline I usually fly, I am not saving any money to do so. But I probably will continue to use the S&S case because I can fit both of my checked bags in the trunk of a Prius, most taxi cabs in my community use a Prius. Getting a big bike box to and from an airport is not convenient if it will not fit in a taxi. The downside of the S&S couplers is that I have to completely disassemble the bike to pack it, and completely assemble it where I arrive. That is an hour and a half to two hours at each end. But, I built up teh bike from parts, thus I am comfortable doing that. If you are not mechanically inclined, that could be a more daunting task.
Can you carry the S&S Backpack case on the bike for a couple weeks? It would not be convenient. My tours have started and finished at the same location. The four side panels are approximately 30 inches (~75 to 80 cm) by 10 inches (~ 25cm), they are flexible but quite stiff and probably should be handled flat. The rest of the bag is a lot of stiff fabric but it probably can be rolled up or folded to a much smaller package. Maybe if you contacted the S&S company and aske them how big the package is if you buy a new S&S Backpack case, that package size will tell you how compact they make it for shipping. I bought mine a decade ago, I do not recall packed size.
I always suggest that derailleurs be removed from a bike before shipping. A steel frame with the derailleur hanger is usually robust and not a problem, but many if not most bikes now have a separate derailleur hanger that is less robust and easily bent out of alignment. Replaceable hangers are also considered a sacrificial component that can break instead of damaging the frame. On my titanium bike, I carry a spare derailleur hanger when on a bike tour. Hangers are brand and model specific.
I think a derailleur system might be a bit easier to pack in an S&S case, when I have packed my folding bike in that case I removed the cassette and packed that separately. A Rohloff hub has a big wide shell and that makes the spoked part of the wheel thicker, more volume is consumed. But as you can see in the photos to that other link, I have done it.
Wheel size, it is a lot easier to pack a 26 inch wheel in a S&S case than a 700c wheel.
Many of my friends will pay a bike shop to pack their bike at the end of a trip. A lot of people go to a bike shop to get a bike box at the end of a trip and pack it themselves. But you are betting that they will have a spare box that is big enough. Several years ago I went to a local bike shop, they did not have any spare boxes. They asked my bike size, said they would call me when they got a box for me. A couple weeks later they called, I drove over there and got the box. (I had a Land Rover, so I could carry a full size box at that time.) And, about a week before my trip I started to pack up one of my full size bikes that did not have couplers, and the box was too small. Made a few calls to other bike shops, and quickly gave up. For that trip, instead I brought my folding bike in my S&S case, I did not plan to do that but that was the simplest solution. I have heard of people using several boxes and taping them together to fit their bike. That is another advantage of my S&S couplers, I know I can pack that bike into the case, procuring a box is one less thing to worry about.
International travel, you almost have to have the bike on the airplane with you, otherwise customs people expect that a shipped bike is an import, which may involve customs. So, that makes packing a bike for air travel more critical. I know several people that have done a lot of bike touring, but have never taken a bike tour outside of USA for that reason.
There is nothing wrong with S&S couplers, as I noted above they are less popular now that some of the big USA airlines have dropped their oversize fees for bikes. In 2016 before they dropped the fees, I brought my bike to Iceland, flew Delta Airlines from USA, I saved $300 (USD) by not having an oversized case for the out and back flights. And in 2016, that much money went a lot further than inflated dollars do now. At that kind of price, you can recover your costs for S&S couplers and the case with very few flights. But now, they no longer charge for oversize boxes if they contain a bike.
Surly used to sell some models with S&S couplers, but I believe that they no longer do so simply because it is less popular in USA now.
Outside of USA, if S&S couplers are less popular now than they used to be, I do not know the reason. I think that aluminum frames can't be fitted with S&S couplers, thus some brands simply can't use them. And I think that more and more companies are switching to aluminum and away from steel. I can have them added to my titanium (derailleur) bike, but it would be quite expensive, so I am not doing that. Instead I just won't fly with that bike.
If you are using panniers, I think S&S couplers are the best option for couplers. I think that one of the tandem bike companies has created another coupler system but I am unaware of any major bike companies that use it. Ritchey for several years has made a Break Away bike. That is a coupled system, but I think the bike is less robust. I have a Ritchey Break Away bike too (mine is badged as a Raleigh) but I would not recommend putting a heavy luggage load on a bike with the Break Away system. I however met a couple in Iceland that were traveling using bikepacking gear, no racks, and they had Ritchey Break Away bikes. But they were staying on pavement and were packed extremely light, where I was also traveling on 4X4 vehicle roads with my Thorn.
I hope I answered your questions, I spent much more time writing this than I planned. A few more notes, below.
No matter how you travel, you probably will benefit from having a luggage scale along. One overweight piece of luggage on one trip can pay for many luggage scales.
One more bit of trivia, if you are camping and flying, airline security occasionally want to see your camp stove. The two times when security has wanted to see mine, they said if they could smell fuel, they would confiscate it. I have not lost a stove, but making a liquid fuel stove and bottle odor free is complicated enough that now I only fly with butane types stoves. More at:
https://www.msrgear.com/blog/flying-with-a-camping-stove/
