Keep in mind that most slippies don't have adjustable pivot screws; the tension has to be dead-nut from day-one and stay that way for the life of the knife.
Also, unlike the populer liner/frame locks that allow the blade to move more or less freely between its closed and locked positions, a slip joint's tang maintains intimate contact with a single backspring that has to hold it (firmly!) open and closed while applying constant pressure against the tang at all times. If the blade travel isn't smooth, consistent and predictable across its entire path - if its "walk" isn't up to snuff - you'll feel it every time you open or close the knife. Throw in a half-stop and a backspring that remains flush in all three blade positions and the knifemaker is confronted with a significant challenge. A slip joint with excellent "walk" and a nice positive snap (or "talk") at both ends is extremely satisfying from a tactile, as well as functional, standpoint.
Frankly, I don't know how the makers of affordable custom slip joints mentioned in this thread manage to do such amazing work for such a reasonable price. No wonder these timeless custom knives have enjoyed such longterm popularity.