Is mixing shellac from flakes generally considered more cost effective than buy commercial stuff?
The flakes have a much longer shelf life than mixed shellac, somewhere on the order of 3 years vs 6 months depending on environmental conditions. That makes the flakes a better value if you can't use an entire can at once. I suspect you won't find much of a price difference. The harvest is really the largest factor in pricing.

You are correct about the customizability. Flakes can be bought in a variety of hues, where as the canned is likely blonde. Being able to precisely control the cut is nice when it comes specialty uses such as refinishing and French polishing. (You know, in case you want to seal away tobacco odors or get a piano-like finish. Or refinish a piano played by a heavy smoker.)