Two big problems:
1. Half of the politicians that make it into parliament are from the party list (which is chosen by the party) and are not voted for individually. I believe much of the power of democracy is the ability to vote people out, and the party list mostly prevents that.
2. There is a lot of talk of strategic voting. I guess all systems can be gamed, but MMP seems to encourage it.
https://elections.nz/democracy-in-nz/what-is-new-zealands-sy... “Every candidate who wins an electorate gets a seat in Parliament. They are called electorate MPs. The remaining seats are filled from party lists. Every party has a party list, which is a list of candidates ranked in the order the party wants those candidates to be elected to Parliament. Candidates elected from a party list are called list MPs.”