As someone notes in the responses:
"Congestion pricing is such a good idea everywhere there is rock solid public transit alternatives. Where there’s not, it just becomes a tax on the poor."
If most of the poorer and working class road users cannot shift their transport times (and typically they can't) and then you don't have good and affordable public transport options for them to take instead then it effectively becomes a charge that the people who can't avoid it pay so that those with flexible lives can enjoy a smoother commute.
Which is to say that it might well reduce congestion but the cost of doing so is lumped on those that can least afford it, rather than those that can. As with most things its a choice of who pays for what to reach the aim. I would prefer that we increase taxes on wealthier people and fund expanded and free(er) public transport with a huge increase in safe cycling routes.