I just googled "Water Softner Drawbacks" and extract from one of the search results below.
Disadvantages of home water softening:
• Can corrode your pipes. The corroded metal from the pipes can end up in your water. This can contribute to elevated lead and copper levels in drinking water.
• Potential health effects from additional sodium.
• Regular testing of the water and maintenance of the softener is necessary to make sure the softener is working properly.
• Negative impacts to the environment from salt use.
• The water used to regenerate the softener beads ends up as waste.
Just add fungal top the search and you will get results for that as well.