See this recent paper: Quantization Degradation in Large Language Models: A Signal–Noise Perspective [1].

  We observe that such degradation varies substantially across these factors: 4-bit quantization usually preserves performance, 2-bit often causes broad degradation

[1]: https://arxiv.org/abs/2608.08188

I created a pruned experts model of the q2 quant, while it gave good performance on limited hardware there was severe quality degradation.