I heard that there may be issues with very low-water WCs, such as the 2-4 liters per flush – i.e. in the sense that you would need to flush it twice to get the effect needed.
1- Does this fact hinder their reliability? Is there any recommended measures to reduce the problem with such WCs?
2- What would be the lowest reasonable low-flush WCs we should aim at specifying without running into this problem?
3- Is there any index other than gallons-per-flush that may help in deciding whether it is recommended to use low-water WCs?