In the video at around 6:45, it is mentioned that adding Shift is basically equivalent to using the Smooth brush. It does produce a smoothing action, not always what I expect.
A. Does Shift + [brush] use the size/strength of the original brush, or of the Smooth brush, or something else? All the brushes might have different sizes / strengths.
I tend to get lots of different results with Shift - I tend to go for Smooth brush instead, for predictability.
B. Follow-up: Is it ever advisable to set "unified size and/or strength" on the brushes? I see "unified size", not sure if that is the default or if I changed it. I know that is kind of advanced, but maybe it could help a beginner?
A. SHIFT + Brush uses the strength of the Smooth Brush, but the Radius of the active Brush.
B. Unified Radius is default and I leave it on, because I need to adjust the Radius all the time, no matter what the Brush, but I never found a situation where I wanted to use unified Strength, but I am not an experienced sculptor, so...
I actually prefer unified strength as well as radius. Perhaps I'm more simple-minded than most but my brain usually thinks in singular strength as I work between brushes. So if I make a strong crease then want to pinch it tighter, when I switch to the Pinch brush my mind still presumes "strong effect". But if the pinch brush is really weak I'm briefly taken out of my mental zone and have to adjust the strength.
That said, I'm sure there are brushes that I primarily use weak strengths for which would make sense to leave low and disable unified strength.
In the end it comes down to preference. Try both and go with whichever best facilitates your workflow.