From a leftist point of view the most frustrating thing about red-green is that they market themselves as progressives and leftists but then just follow the same neoliberal economical politics as everyone else. It's deeply ironic that the reforms that did away with large swathes of the welfare state and labor protections were implemented by a red-green coalition government, not conservatives or the FDP.

The lesson most voters take away from this seems to be that leftism is just false advertising and if you vote for leftists you just get the same politics but more dishonest, not that the "leftists" they keep voting for aren't actually interested in leftist politics (though I can see why Die Linke might be unappealing as they're a headache even if you see them as the only viable option).