That's a tough one to get your head around if you don't choose an exact moment and set of parameters.
To look at a straight descending side slip such as one might normally do at the end of the final turn. One wing is low and it's full area is presented to the airflow. The other wing is partly blanked by the fuselage. To maintain a constant bank angle another force must be present to balance the different amounts of lift each wing produces. This is, of course, the reason for the aileron in the opposite direction to the rudder input.
This is, perhaps, where the discussion really gets interesting. The aileron then, increases the angle of attack of the one wing and reduces that of the other.
You may prefer to say that the aileron increases the lift co-efficient on one side and decreases that of the other but then one must consider the three different lift co-efficients (that of the plain sections of both wings and those of the two parts of the wings where the ailerons are fitted) and their respective maximum co-efficients of lift.
This complicates the discussion unnecessarily for the purpose of the stated argument as when the pitch angle is increased (up elevator), the wing stalls from the outboard aileron on the side of the applied rudder.
Regarding your discussion of the wisdom/safety it should be noted that the most common error in performing a slip is the failure to raise the nose enough which results in a higher speed and consequently lower angle of attack.
Quite alarmingly high rates of descent can be achieved but are also quickly recovered from by centralizing the aileron and rudder.
A thoroughly safe and extremely useful technique and an essential one for many taildragger types with limited forward vizibility in the landing attitude.