Mandatory minimums are a problem. Judges lose discretion to tailor the punishment to the specifics of the case. Minimums may be pushed unreasonably high so politicians can claim to be "tough on crime." (This happened big time in the US, starting with the War on Drugs in the 1970s and continuing through the 1990s.) Both of those lead to more people in prison longer than they should be.

Also, at least in the US, not all crimes carry mandatory minimum sentences. This gives prosecutors a new source of leverage:

The use of mandatory minimums effectively vests prosecutors with powerful sentencing discretion. The prosecutor controls the decision to charge a person with a mandatory-eligible crime and, in some states, the decision to apply the mandatory minimum to an eligible charge. Rather than eliminate discretion in sentencing, mandatory minimums therefore moved this power from judges to prosecutors. The threat of mandatory minimums also encourages defendants to plead to a different crime to avoid a stiff, mandatory sentence.

https://www.sentencingproject.org/fact-sheet/how-mandatory-minimums-perpetuate-mass-incarceration-and-what-to-do-about-it/

Mandatory minimums can also lead to significant racial disparities. The linked article cites an example of very different minimum sentences for different drug offenses, leading to a sharp rise in incarceration rates for blacks but much less so for whites.