Kenneth Arrow proved, in his famous impossibility theorem, no system can be guaranteed to give a reasonable outcome on several criteria all the time:
In short, the theorem states that no rank-order electoral system can be designed that always satisfies these three "fairness" criteria:
- If every voter prefers alternative X over alternative Y, then the
group prefers X over Y.
- If every voter's preference between X and Y remains unchanged,
then the group's preference between X and Y will also remain
unchanged (even if voters' preferences between other pairs like X
and Z, Y and Z, or Z and W change)
- There is no "dictator": no single voter possesses the power to
always determine the group's preference.
But that doesn't mean that some practical systems are not better than others or mostly work well.
But there are specific undesirable outcomes promoted by first past the post systems. Like tactical voting. In the UK, for example, many Labour (left wing) voters may vote LiberalDemocrat (centrist) to out a Conservative (right wing) candidate which some may think is undesirable as it blurs the indicator of what voters actually want in a government.
Single Transferable Vote systems in multi member constituencies (as used in Ireland and some elections in Northern Ireland) vastly reduce the incentive for voters to misstate their "true" preferences as the selection process for winners does a good job processing the ranked preferences in aggregate (not perfect as Arrow showed, but far better than in an FPTP system).
Practical experience shows that voters do not have big problems understanding such systems and that, in practice, they are better at representing their preferences than FPTP alternatives.