The thing is Labour was only ever going to govern with NZ First. It's not good enough for them to say "our coalition partner stopped us from doing this" when they were always going to be a coalition partner. There simply wasn't going to be a Labour government without NZ First.
The idea that you can promise whatever the hell you want, regardless of it being something you can actually implement in government, significantly undermines the public's ability to cast an informed vote. Sure, it's happened before, but 2017 was particular egregious given what was promised, the timelines for it and how quickly those same policies were abandoned.
I don't agree with this at all. Political parties should describe what they would do if they were elected to govern. If they have to form a coalition, because the electorate chose that mix of parties, then so be it. The electorate has chosen to moderate Labour (or Nationals) ability to implement their manifesto.
Political parties can't dictate before the election what percentage of the vote they will get or what potential coalition parties will demand. The amount of power a coalition partner has is proportionate to how much they are required. The Maori party or Act got one or two token policies under National as they were both fairly insignificant and barely required. NZ First gets a lot more say this time as they had two parties to play off against each other.
If the voters haven't figured out what MMP means by now then it's not up to the political parties to tell them. It serves the purpose for which it was chosen - it moderates extreme actions by political parties and provides a more even representation of the countries views.