@Oldwearer There is no difference in what can be achieved through a cut-in with a stock piece to a custom made one.
The question is whether a stock piece matches your own hair in density, colour & ventilation, as well as a custom made piece would.
Some people are lucky enough to have fairly standard specs and so a stock piece is viable.
If, however you have specific requirements such as percentages of grey, a certain shape of front contour etc. then you are best to order a custom and send in a hair sample, to get the perfect match.