I think most Audis and Volvos work better as estates but that is because they have been building “lifestyle” vehicles for the middle classes for donkeys years. Their markets are people who do outdoor stuff and who have dogs and move antiques etc (volvos!) as well as families.
Audi A4 and S70 look terrible but the A4 Avant and V70 look so much better.
Saloons do seem to be popular with certain types – men in their 20’s who like sending their disposable income on their cars and older men who see saloons as “normal” and don’t “need” and estate. Saloons are generally cheaper 2nd hand due to being common company cars hence them being good purchases for people wanting a car in the A4/3 series segment with a 2 litre engine and lots of toys that you dont get on the level below.
Saloons are generally cheaper (less glass, smaller tailgate, lower rated suspension and a bit more secure n so popular as company cars. They are quieter for people in the back and less weight high up makes them better handling.
Saloon versions of what we buy as hatchbacks in the UK are popular in hotter countries I think due to the boot staying cooler and being a good place to put your shopping. I think hatches are very popular in the UK due to looks (we are more obsessed with cars being a status symbol) and reduced length for easier parking and possibly being a bit more sporty due to less weight/overhang or maybe that is just the looks are sportier.
Boot size for an estate is under the load cover + the whole volume with the seats flat and parcel shelf removed. I think my estate is 600 litres under the parcel shelf and just under 2000L (2 cubic metres) with the seats down and load cover removed. The saloon version has a very similar capacity to the first figure as it is the same length for my car (a lot are not) but obviously you have the very restricted access but I guess the thing you need to be able to do is fit luggage or golf clubs. If you need to slide in big 1m cubed boxes you get the estate.