158's had a modification so that they didn't dump on the track anyway.Surely the big issue with toilets is the tanking- for years "dump on the track" toilets could function endlessly and maintenance plans weren't needed to allow for tanks to be emptied. All of a sudden trains now need the tanks emptying at the end of each working day pretty much. Even if all units return to depot that's still a logistical challenge, and with units outstationed overnight away from the home depot that challenge becomes a nightmare.
The vacuum loos don't need as much water to function so that means less weight and less need to fill the water tanks so often along with more space for a bigger waste tank (potentially). If they're unreliable it isn't necessarily a flaw with the concept, just the execution.
In terms of the vacuum loo concept, I'm not convinced it will ever work adequately where it is the only toilet on a unit stuck on a long diagram in the middle of nowhere, miles away from a depot.