Robin 400,
Helicopters tend to have more attitude changes than fixed wing especially those with rigid type rotorheads.
The system on the EC135 is well proven and employed on many similar types. One big longitudinal placed tank feeding two independant supply tanks using a "Full-Feeder" tank principle incorporating "overflow" connections at the top of the suplly tanks feeding back into the main tank.
As the fuel level gets down to the lower levels, significant changes in longitudinal attitude require some carefull management of the fuel transfer system. This is normal and ussually the pilots that fly these types are fully appraised of the system operation and limitations and how to manage the situations as they arise from say, prolonged hovering.
Failures create warnings. Low Level indications are nearly always independant from contents indications (as in the case of the RED LOW FUEL caption in the EC135).
Ultimatley, providing the crew are alert and the checklist is correctly formatted, a landing should take place as soon as possible (nearest safe site) if the backstop (RED) LOW FUEL warning illuminates, regardless of any other contradictory or misunderstood indications.
I would go as far to say that the system is as robust and simple as it can be while employing more than sufficient redundancy in indications and warnings to enable a safe landing to be made under all conditions provided the RFM actions are complied with.
Of course in the case of this accident we do not yet know if any of this is relevant or significant but it is noticeable that the 95 Litres recovered from the fuel system so stated in the AAIB report is similar to the KG value that could get trapped in the main tank as unuseable fuel under certain failure/switch selections.
It is also worth noting that the EC135 worldwide fleet has completed over 2.8m flight hours.