For toilet bowls, I’d imagine that the newer vacuum technology may cause them to smash, and even if not, they’re easier to vandalise, can you imagine if toilets went out of order because someone smashed the toilet? It wouldn’t look good, especially now that retention tanks are needed resulting in out of order toilets much more regularly.
Vacuum WCs with china pans are in widespread in the shipping trade and can alo be found on railways overseas as well. The problem with smashed pans would be no worse than the problem with smashed pans on trains before vacuum WCs were introduced: and given the age of many of those, it does not seem to have been a great problem. They do, however, use more water which is likely to be the deciding factor.
Porcelain toilet bowls are also significantly heavier, as well as being more vulnerable generally to chips and cracks, which makes them unhygienic as they can't be properly cleaned.
The china used in WC pans is coated with 'vitreous china' to form a non-porous, glass-like surface: they are resistant to chipping and cracking in normal sanitary use, which is why they are the usual material for these products in everyday life.
As for weight, the relatively small size of washbasins and WC pans makes the weight insignificant in the grand scheme of a carriage, or even in relation to passenger luggage. The weight of a complete vacuum WC, not just the bowl, is likely to be roughly the same as a washdown WC pan: an Evac Compact vacuum WC is 18kg, a domestic wall-hung porcelain WC is typically 16 to 20kg. Aircraft use a central vacuum system which significantly reduces the weight per pan.
However...
Metal ones work fine for the purpose. It's other elements of design that aren't great.
Indeed: as long as the shape is suitable, stainless steel is a perfectly good material for a WC or washbasin, and many vacuum WCs are given an extra non-stick coating to reduce soiling further, which would be harder with porcelain.
The relative distance between WC and basin in accessible facilities is set by law, but poor angling of walls and poor design of integrated basins in combination with that regulation makes for awkward spaces that are harder for passengers to use in a particular design of accessible WC module that I keep finding on trains.
I have mentioned my dislike of integrated washbasin units on trains before, but repeat it here: my particular bugbear is the hiding of the tap spouts behind the mirror, high above the basin, whereas on continental trains and Eurostar the tap spouts are lower, separate and visible - like traditional taps - which makes them much easier to use than waving your hands blindly under the mirror and then trying to keep your hands stable in an invisible target area on a rocking train. And having to hold your arms horizontally to reach a spout deep under the mirror without being able to bend over the wasbasin, rather than bending your body over the basin and bending your arms slightly to hold your hands under a tap, is very awkward for tall people while trying to keep your balance on a moving floor.
