No doubt duff static ports will exacerbate the ASI readings in slip.
However the reason why it underreads in a slip is not primarily due to static port problems but because the pitot is not aligned with the airflow in a slip.
It is difficult to stall in a slip.
Of course slipping on finals is an extremely dangerous pastime and should only be done by demi-gods not any ol' ATPL/heavy metal driver.
Come on get real!
As others have said, with proper tuition and practise a slip is perfectly safe and useful.
FD