New Zealand changed from first past the post to MMP, a proportional system that leads to multiple small parties being elected to Parliament. The two biggest parties then have to appeal to small parties to form a coalition. In practice this leads to the party with the most votes being forced into opposition by back-room deals between the loser on election day and the minor parties. The second past the post party can lose the election but win power by bribing the smaller parties to support them.
The last coalition was between Labor (Further left than Democrats) and New Zealand First (further right than republicans), whose voters expected them to form a government with the Centre Right National party. Proportional voting leads to minority parties playing King-maker, the tail wagging the dog. This gives grossly unfair results and unstable coalitions that no-one voted for. I use New Zealand as an example, but the same is true of all proportional voting systems around the world, from Israel to Italy.
First past the post turns out to be the fairest system, and delivers clear winners and clear losers most of the time. With Proportional systems you don’t know on election night who won: you have to wait weeks for the backroom deals to be done, to cobble together a government made up of multiple parties.