The briefing document of the European Parliament offers a quantitative and comprehensive view on the issues with Nord Stream 2. This excellent figure alone explains a lot:

It may be interesting to realise that the points this document makes are underrepresented in popular press such as BBC. These concerns are discussed in measurable terms:
- The existing capacity is enough
- Nord Stream 2 harms the diversification of routes (Polish/Ukrainian paths reduced, Germany becoming the main hub)
- There is a documented history of using the gas infrastructure for political purposes
- EU bodies warned on political and security risks